How Should I Word A Letter To Ask Permission To Take Pictures Of Preschool Children For An Art Project?

I work with a preschool class and I want to do some simple projects for the parents of my class and for some of them I would like to take a picture of the kids and have them frame it for an art project along with a class picture. I will give all the parents the picture of their child, but would like to keep a class picture and give one to each child. I am not sure about legality, and how to word it so they sign something that will cover my behind about taking pictures of the kids. None of it is sexual in any way, but I have heard you need permission from parents to take pictures of children, even at school. This is my first year as a preschool teacher, so I haven’t had the experience so any help would be great! Thanks!