Richard Shilling, a land artist himself, and Julia Brooklyn have written a book, called Land Art for Kids - On the Beach aimed at getting 3 to 8 year olds outside and creating nature art. If you follow the link, you can preview many or all of the pages. It looks like it would be super inspiring for kids and parents alike.
I'm buying the book as a Christmas gift for Maia. (For me really, but I hope she likes it too!)