I wouldn't have kits younger than 8 weeks old in a cage where they could fall any farther than 18"...they tend to be a bit clumsy/careless. This usually means the cage can be no taller than 18" as some like to climb up the sides/door of the cage and then just let go at the top.
My breeding cages all have one shelf 8" off the cage floor. Kits are born in these cages and I haven't had a problem with having a shelf, not with kits falling off it or with mom's using it to escape/ignore the kits. The kits can usually get on the shelf on their own by about a week old. I'm sure they'd be up there sooner if I had wire cages for them to climb the sides of.