I don't own these characters aside from the next generation children.
Phineas was waiting for his wife Isabella to come out of the bathroom. If what they suspected was true a new member would be added to their family. Isabella came out holding a small stick of plastic, it was a pregnancy test. She showed him the result, it was positive.
"So that means?" asked Phineas hopefully.
"Yup, I'm pregnant again," said Isabella.
Both of them were overjoyed. They decided to wait a while before they told their friends and family, but they wanted to tell their one-year-old son right away. He was still young, so he didn't talk much, but he seemed to understand what his parents said and always responded to the best of his ability. They went downstairs and found Sean playing with building blocks, unknowingly being supervised by an aging platypus.
"Sean," said Phineas.
Sean looked at his parents and smiled.
"Mummy, Daddy."
He ran to hug them, after which they all sat down on the couch.
"Sean, we have some big news," said Phineas.
"News?"
"Yeah, your mother and I are gonna have another baby," said Phineas.
"Baby?"
"It means your gonna have a little brother or sister to play with," said Isabella.
It took Sean a while to think of the right words.
"Sean, be big brother?"
"That's right, you're gonna be a big brother," said Phineas.
Sean smiled.
The next few months went on without much surprise. Isabella suffered the same issues she had when she was pregnant with Sean, like overeating and being sick during certain times of the day, but other than that their lives weren't too eventful. As time went on, Sean kept asking his mother and father questions about pregnancy, especially when he started to notice Isabella's growing belly. When they explained that's where the baby was, he feared his mother may have eaten his baby brother or sister. They eventually convinced him they had done nothing of the sort.
Over time they told all their friends the news and everyone was overjoyed for them. Even Phineas' sister-in-law, Vanessa managed to drop in for the baby shower, despite the fact she was currently promoting her debut album.
"So, child number two? You excited?" asked Vanessa.
"Yeah, I haven't been this excited for a while," said Isabelle placing her hands on her growing belly. "Have you and Ferb ever thought about having another child?"
"We've talked about it, but we've already got our hands full with Rose and my music career's only just taken off, so we want to wait a little longer before we have anymore," said Vanessa.
"I can understand that," said Isabella.
Nine months passed by surprisingly quickly, during which time they celebrated Sean's second birthday. He was now speaking full sentences and was even able to read a little.
The day came when they needed to take Isabella to the hospital. Sean was worried, he had never seen his mother in so much pain. When he tried to ask his father what was wrong, he simply told him that he would tell him when he's a little older. Sean wanted to go to the hospital with his parents, but Phineas and Isabella both knew it was going to be a long stressful day. Before going to the hospital, they stopped at the house of Linda and Lawrence, Phineas' parents.
"Thanks for taking him on such short notice," said Phineas.
"No problem honey. We always love spending time with Sean," said Linda.
"But Daddy, I wanna go with you and Mummy," cried Sean.
Phineas looked his son in the eye.
"Buddy the best thing you can do right now is stay with Granma and Grandpa, so we know you're safe. Can you do that for us?" asked Phineas.
In truth Sean still wanted to go in the hope he could be of some help, but if this was what his parents needed him to do, he felt he had to agree.
"Ok," said Sean.
"Thanks Sean," said Phineas patting him on the head.
The rest of the day was a nightmare for both Phineas and Isabella. Isabella was in labour longer than she had been with the last birth and no matter how much time passed, the baby did not seem to want to come. Eventually Phineas was asked by one of the doctors if they could talk outside.
"We're gonna have to give your wife an emergency C-section," said the doctor.
"A C-section? Why?" asked Phineas.
"Because we think it's the only way we can save the baby," said the doctor.
"Can I talk to Isabella first?" asked Phineas.
"Of course."
Isabella agreed immediately.
The next day Sean was eating breakfast with his grandparents when they received a phone call. Linda went to answer it and when she returned, she had a bright smile on her face.
"Well Sean the baby's come," said Linda.
"Baby brother or sister?" asked Sean.
"Your dad won't say he wants us to see for ourselves," said Linda.
"But I wanna know," cried Sean.
"Well then, how about we get going to the hospital so we can see," said Lawrence.
"Yes, let's go, go now," said Sean.
They got in the car as Sean was growing more impatient by the minute. They found Phineas waiting outside Isabella's room. Linda and Lawrence decided to let Sean go first, as he really wanted to meet the baby. Phineas took Sean inside, where he found his mother sleeping in a bed and a baby sleeping in a cradle. Phineas put Sean on a stool so he could see better.
"Sean, meet your baby sister Teresa, but you can just call her Terri," said Phineas.
Sean looked at the baby in the cradle. She had black hair like their mother, with a triangle shaped head like their father.
"She's my little sister?" asked Sean.
"Yep, here she is Sean," said Phineas.
Sean reached in and poked the sleeping baby. He laughed when he started poking her on the nose.
"She looks like you daddy."
Phineas laughed at this statement.
"Yup, she's got the famous Flynn Triangle head."
Sean then notinced the card which showed Terri's name. Sean pointed at it.
"T, er, esa. Viv. Vivi. Vivi," Sean struggled to pronounce the whole name.
"That's her name buddy. She's Teresa Vivian Flynn," said Phineas.
"Like Grandma Vivian?" asked Sean.
"Yup, we named her after Grandma Vivian," said Phineas.
"Why does she have two names?" asked Sean.
"Buddy most people have two names," said Phineas.
"Do I?" asked Sean.
"Yeah, your name is Sean Ferb Flynn," said Phineas.
"Like Uncle Ferb?"
"Yeah, just like your Uncle Ferb."
Sean then directed his attention back to the baby. Phineas put his hand on his son's shoulder.
"Sean."
The child looked up at his father.
"As Terri's big brother, I'm counting on you to protect her while you guys are growing up. Do you think you can do that for me?"
"I will daddy I promise," said Sean.
"Good boy," said Phineas.
Sean then noticed his mother was still asleep.
"Why is mummy sleeping?" asked Sean.
Phineas felt Sean was a little to young to hear about the procedure Isabella had gone through. Nevertheless, he decided to be as truthful as possible.
"Your mother had a long day. Giving birth isn't easy," said Phineas.
Having seen what his mother had been like the day before, Sean did not question his father any further.
A few days later they got to take Terri home.
As Phineas put Terri down to rest in the new nursery, he gave her something that would hopefully help her feel safe in the years to come. He had given Sean a similar one when they first brought him home. It was a stuffed Platypus, which he placed at her side while she slept.