Sounds like you have a good beat on what's going on. Hopefully it is just a stage and what you describe seems to be pretty normal, especially for HG(+) girls who want to fit in. At least that's been my experience. I would be more concerned if she's still doing it at 7 or 8. At that age, I'm thinking the baby talk will NOT be accepted by peers, if she's still doing it and possibly even getting teased for it, I'd definitely look further into it.