Melissa,
This a frustrating problem for many foster meowmies. They are still babies but mom should be teaching them the use of the litter box. Have the kits been to the vet yet? There may be a medical issue involved with their behavior.
It’s good you are using an enzyme cleaner. How many litter boxes do you have for them? I know they’re kittens but the general rule is one box for each adult cat. Also, changing the type, smell and location of the litter is going to confuse them and then they won’t know where their potty is in the house. If they’ve already used the floor, it doesn’t do any good to put them in the box. Within 10-15 mins. after they eat, put them in the litter box. Hopefully, there is some sample for them to understand what to do in there.
If you don’t have room for that many boxes and most people don’t, set up a couple more boxes where they can find them. You select a non-clumping, unscented litter and use that exclusively. The scent in litter is to appease humans not the cats.
I hope something here will help you. Good Luck
PG