Can you put a small box in there for them to snuggle together in?
I agree with sarther2, they won't necessarily urinate on the couch, just make sure there are litter boxes that are easily acessable to kittens (low sides) with non-clumping litter inside. Underneath the sofa throw you can place water proof mats such as baby crib liners, twin bed water proof mattress pads, etc. that would absorb any moisture from the kittens just in case.The gate plan sounds good. I don't think there's much you can do about the couch other than spreading towels over it.
I had a brand new couch last year when my kittens were little. They never peed on it even once.