B.W.
That's cool that you took the time and made the ID cards. I would definately NOT give them to the kids. I've read stuff that says you shouldn't put their names on the outsides of their backpacks, or on their shirts, or anything, because it would give a potential "stranger" the ability to call them by name. It might trick the child into believing the person knows them or their parents. And it only takes a minute or two to steal a child.
Also, at 7 mos. and 2 yrs., they are way too young to hang onto them. You don't want them to land somewhere else, and get picked up.
So, for now, keep them with you guys -- or if you want to make duplicates, give a set to the grandparents.