White Sands monument has never been very populated with visitors when we've been there, and we often see only a handful of others.  So when anyone shows up nearby, it's interesting to watch them.  In this case, a young family showed up while the dune jumping was going on, and their two little boys appeared to be about three and about five years old.  We offered to throw the kids off the dune with their parents' permission, but they declined.  The older boy, however, darted right on over when he heard us, but the younger one hung back at the truck.  He was the probably the smarter one!