You might go out with the kids if that is possible and let them pick out a little something. I would make it something inexpensive and definately from the kids. A nice gesture on your part but don't let her think you had an influence on the gift. You just want your kids to have something to give her.
Just my two cents worth. I wish my H would do that. He never takes the kids, he just picks up something. I doubt he will this year but that is okay with me. I will treasure what the kids make from school.