Are the trays low enough for the kits to climb into? We used seed trays for the first month or so.
Lifting them into the litter tray regularly helps.
Keeping the trays clean is important.
Making sure you clean up all the accidents as soon as possible and use an antibacterial spray on the patch - we use dettol surface cleaner spray, which isn't overwhelmingly smelly but does keep them away from the same spot.
For the current pair of kittens (now 2yo) we set up a 6'x6' kitten area in the lounge with food and a litter tray at opposite ends, and a plastic party tablemat covering the carpet. That caught most of the accidents.
We put another litter (seed) tray in each room.
Should be fine within a week or two. Yes, fraid so.
If you reduce the count of litter trays, put a large box or something to cover the location where it was or they might continue doing their business there. Don't go under one tray per kitten though.