It's hard for me to tell in the pictures, but if they are all losing hair, then they may have a an infection of some sort. Healthy kittens with good diets do not lose hair. I think you may want to call your vet. It does not look like ringworm though. Let us know what the vet says.
Their hair grew back within a week or two of that posting. Then I took them to the vet to get worm medication and get the feline vaccine and a check up. This is a photo of Tuco, Angel Eyes, and Blondie from July 16.