Dog eating cat poop out of litter box?

ok so i have a one year irish setter, and 2 litters of kittens. (plus moms) i have one tall litter box for mamma cats in the bathroom across from where i keep the kittens. i also put a low litter box in the room with the kittens so that the mamma cats will poop in that to show the kittens where to poop. one of the litter of kittens are 4 weeks, and the other litter is 3 weeks. the problem is the mamma cats wont poop in the lower litter box. and my dog is eating the mamma poop, out of the bathroom. what do you think i should do to make them poop in the low litter box?