mama cat won't stop using kittens litter box even when hers is clean

My foster mama is driving me crazy because she won't stop making a huge mess with kittens little litter box. The other day I had to clean off one of the kittens because they stepped in her massive ( compared to them) poop. Even when her box is totally clean she uses theirs for some  bizarre reason. She was an outside only cat until I took her in. Her box is too tall for them to get into so I set up a much shorter and smaller box for the kittens. Mama is making a mess with poop piles and flinging litter all over outside the kitten box when she uses it which confuses them so they are now sometimes peeing next to the box because hey there is litter there. 

I'm at my wits end and I can not constantly watch the litter box to scoop it the second she uses it. 

She was perfect with using her litter box before I gave the kittens their own.

Now, when she has tow boxes, and uses the kittens for poo, how is she using her own litterbox?

My main theory at this moment, is, outside living cats do seldom poo and pee in the same place. Of course, they do seldom it at the same occasion, so it is natural they do it at different places.

Home cats learn to use one litter.

So a good idea with adopted homeless is, to give them two different litters...

Another possibility, is she simply marks her revire in kittens litter too...  Just to be on the safe side. 

Good luck!

She using both hers and the kittens for both pee and poop. In fact I have scooped both boxes multiple times today from her usage. There is no rhyme or reason I can tell. Two of my other adult cats have separate boxes for each due to their preference to only pee in certain boxes. But this girl is confusing me. You might be right about her marking it for the kittens but it's still annoying. I don't want the kittens stepping in her poop and getting it all over themselves like before.

You can't keep mama cat from using the littler box.  I would just keep it clean so the kittens don't get dirty.

It struck me "getting all over themselves"? So it is rather rare or even a diarrhea? It is perhaps this which is the problem, she has more or less a diarrhea?   The normal poo is quite fast.

I replaced the kittens box a couple days ago because mama had knocked it over plus I didn't think of the fact that the kittens might all need to use it at the same time and only 2 fit in the box at the same time. So now it's two big boxes and it's working out great.