I wouldn't go for a baby. I'd say someone at least able to talk, I think I'd go for 8-10 years. Of course that requires a backstory for the child, though! Some reason why she's better off with Frank and Matt and not the police/her actual family (if she has one). Maybe she’s an orphan who ran away from the orphanage for X reason? And Matt doesn't have the heart to give her back to the foster system (?).
Random suggestions that came to mind.