As Ela says, the kittens were too young to leave their mum, so it sounds like they haven't fully got the grasp of litter trays. She could prob do with putting a few trays down, to help the little girl and also when she has to clear up an accident, rub the tissue round the tray so it smells like a toilet to her. A covered tray might be too confusing at that age, and she might also not like the litter, we always tell people wht they are using so they can get the same. Good luck with her.