This is a simple trick!

What you do is this:

Place the litter box near the toilet for a few weeks so the kitten will get used to doing their business in the bathroom.
Take an old toilet seat (with no cover) and place it so that it sits over the litter box. This will train the young cats to aim through the hole in the seat.
After about a week; use some sort of tray or pan to place under the toilet seat (within the bowl) so that the toilet seat will completely close over the pan or tray.
Now show the cats where the litter is by placing their paws in the sand! After some time you can slowly remove the pan/tray. The cats will just think you never filled it with sand and will use the toilet as long as there is no prior waste
By taking the litter pan away; they are forced to use the toilet because that is the only place that had the litter.
They should now be familiar with the toilet seat...