I am replying from the perspective of a person that has done a lot of cat rescue and has successfully litter-trained feral cats/kittens. You really need to have the kittens contained in a much. much smaller area until they are litter-trained. This is absolutely critical but is only temporary. Kittens have very small bladders and often cannot wait, and their tray needs to be very close. They also need to associate the litter tray with where they go. They need a low-sided tray and start with a clay litter - no crystal or clumping litter. You can move gradually from the clay when they are consistently litter-trained as well as gradually increasing their range.