I am a Mother of a 5 year old and an 8 year old and I am so glad to read that you agree that 7 is just way too young for a phone. I probably wouldn't have asked for it back either. It sounds like you are disappointed that it wasn't used for it's intended purpose, and maybe even feel a little cheated since the child's Father is using it...however...just coming from the standpoint of a Mother who has children with very generous Uncles...I would always ask before giving a gift of this nature. That way you can be sure if the parents will find it appropriate, and if they tell you before you have given it, the child won't be upset and you won't be out an expensive gift. Used or not.
I can tell your intentions are good here, but just as a Mother I think asking ahead will save you grief and I believe she will really appreciate that you are giving her the choice as a parent to kindly decline. Know what I mean? I think asking for it back might just create unnecessary drama...I would let it go and just tread carefully in the future. :)