I'm not completely sure, because I don't tag and am not very savvy with such things, but I don't think you can use children's names to tag. You have to use the adult's name, (or whoever has a FB account).
For example, friends tag me, but when I click on the picture, it's just their kids and my son. All kids in the picture. Of course, none of those children has a FB account, so it's me she tags. It sounds like I'd be in the picture, but it's just the kids.
If they are friends on FB, she'd have to tag him so he'd know that there's a new picture of his daughter. She can't put the child's name on the tag.
Now, she could just create an album and direct him there, but a lot of people on FB seem to be into tagging and think that's easier.
I hope you don't get too annoyed at this and don't make it into an issue. Anyone who really knows you realizes that your husband is with you and that you're probably not going on vacation with the ex. (Sorry if that's not even entered into the thought process but just wanted to add it in case).
That's the best explanation I know. Hope it helps.
J. F.