They were three when they first met.
Their mothers had taken them to a little wizarding park, and then left the two to their own devices. Her mother was busy with her baby brother, now almost a year old but still fussy. His mother was speaking with some old friends. Had the two mothers seen their children approach each other, they would have pulled them apart, and this story never would have happened.
But the mothers didn't notice their children. So this story began.
He was playing with magic shovels in the sandbox. If you asked them nicely, they would dig for you. She went up to him.
"Can I use a shovel pease?" She was ever so polite, and quite linguistically talented (though she always said please wrong).
"Of course, here. Don't forget to be nice to it, or it won't work." He was a little better with pronunciation, though her vocabulary was larger.
They dug in the sand for a while, then began to build a castle together. When the castle was done, they sat back to enjoy their masterpiece.
"I'm Rose, Rose Weasley. What's your name?"
"Scorpius Malfoy, but most people call me Scorp."
"Well, Scorp, you can call me Rosie." She smiled widely at him. He smiled back. "Do you want to be friends?"
"Of course!" came the reply.
Just then, Rosie heard her mother call for her. "Oh, I'm sorry, I've got to go. It was nice to meet you!"
"Can we see each other again?"
"Of course! I'll ask mommy to come back to the park next week."
"Me too."
And she left him smiling, thinking about the pretty little girl who was just as smart as he was. He had a new friend.