"I'm sorry dear, nothing is going to get you while you're in the TARDIS. There aren't any boogeymen here. But.. I'll put a night light in your room so that you'll be okay, okay?" Not to mention that maybe she'd put the girl next to their room so that she would be well protected.
Donna nodded at her before putting the spaghetti in to boil. "I will, I promise. And yes, I do like ponies very much! Do you?"