Author's Note: Had this plot bunny in my head for weeks. Inspired by my eight year old sister, who likes to sleep in my mom and dad's bed on occasion.
An attentive ear can hear the sound of even the most quiet footsteps. Sherlock had an attentive ear, and there was someone trying to sneak across the living room floor on tiptoe.
Sherlock was reading a medical journal of John's for lack of anything better to do, and upon hearing the pitter-patter of tiny footsteps, he closed the journal and looked up at the little, blonde girl attempting to creep upstairs without Sherlock noticing.
"Rosamund, it is past your bedtime." Sherlock said.
The little girl froze on the third step. "How'd you hear me? My footsteps were very quiet because I'm on my tiptoes." Rosie spoke in a stage whisper that carried across the room.
"I have good hearing." Sherlock said, raising an eyebrow at the four-year-old as if to say no one moves in this flat without my knowledge.
Rosie looked up the stairs forlornly and then looked back at Sherlock.
"I wanna sleep in daddy's bed." She said softly. "My bed is lonely."
Sherlock observed the child's messy plaited hair and drowsy eyes. She had been waking up in the middle of the night and trekking up to John's room to sleep almost every other night over the last ten days. Sherlock usually managed to convince the already headstrong at four-years-old girl that her father needed sleep so she shouldn't disturb him, and she had to sleep in her own bed.
Tonight, Sherlock didn't bother. He'd read on a couple parenting websites—John mocked him for reading these, but a lot of them were helpful in taking care of their young charge—that most children find it comforting to sleep in their parent's beds on occasion and as long as the child knew that the arrangement wasn't permanent, it was alright to allow it.
"I'm sure your father won't mind one night." John probably would mind, but Sherlock would deal with that in the morning.
Rosie smiled at Sherlock, "No, daddy won't mind. I go to his room, now." And she waved at Sherlock and continued up the stairs.
A couple hours later, at around 2:00 am, Sherlock carefully opened John's bedroom door to see how the father and daughter were doing. Rosie was curled close to John's side and John looked content in sleep, one arm cradling his daughter.