Most phones intended for younger children now have a service that allows you to see on a map where the child is, it is for safety. The only time it would be considered an invasion of privacy is if she can prove he gave it to the child to track her movements not the childs. If he has instructed the child to take pictures of mommy and send them to daddy it is an invasion. If he is trying to prove she is unfit by where she takes the children it is not considered an invasion if the children might be in danger.
How do I know this? Steven has had a cell phone since he was 9, he is 17 now. I travel a lot and like to be in contact with my child if he's not with me on something like a business trip. My ex husband tried to sue me saying that I gave Steven a cell phone to keep track of where he is not Steven. The phones didn't have camera's then and only a triangulated locater. But if I called Steven you could hear him in the background screaming at Steven not to tell me where they were that is was none of my business.
My lawyer stood up there and defended me against this garbage and defined very well what is invasion and what is not.
It comes down to the intent with which the child was provided the phone.
If she isn't doing anything wrong or taking them where they shouldn't be she should just get over it and ignore it.