We have our first kittens, a brother and sister about 11 weeks old.
They came to us litterbox trained and have been good about that, but
they love to play in the litterbox.  We want to train them out of
playing together in the litterbox, but don't want to train them out of
using the litterbox as a litterbox!

Note that the litterbox is one of those "cave" types with a hood.  Is
that the problem??  Is it perhaps that they just want an enclosed
space to play in?  (Taking the hood off now to see if that helps!)

Any suggestions?