My two nieces are very different in this respect. The younger one loves being in open-toed sandals: she wants to wear them at the first hint of spring, wishes she could wear them to school, goes barefoot in the garden, and would probably go barefoot everywhere if she was allowed. (Her parents discourage flip-flops, except for the beach and pool, thinking that it doesn't do children's feet good to walk in them.) But the older one hates sandals of any kind, saying that they hurt her feet, she likes her toes to be protected otherwise she stubs them, and she lives in trainers through the summer, and says firmly that she will never wear sliders and socks, like teenagers do. She doesn't like Crocs either.
Come to think of it, I didn't wear sandals much as a child, and never flip flops. I always liked trainers; my concession to summer was to wear them without socks. I didn't like sandals until I was about thirty, and I can't walk in flip flops.
What do your children like on their feet in the summer?