My 4 y/o Grandson was extremely happy to have "flip flops" like Daddy's only cooler 'cause they had dinosaurs on them.  Unfortunately after wearing them for about 2 hours his little feet were hurting.  His Mom thinks that the heel strap may be too snug and is forcing his feet too tightly into the spot between his big toe and his second toe causing uncomfortable pressure.  He didn't have blisters, just foot pain.  It may be that his little feet are just too young to hang onto the shoe without the heel strap.  He was very disappointed and kept trying to wear them and paid the price by having a bad nights sleep with foot and leg pain.  We may try to make the elastic strap a little bigger so that it only hold the shoe in place and adds no pressure to his feet. Mom has put them away for now.