My kittens are now 10 months but we kept them inside from 8 weeks to 20 weeks when they were spayed/neutered eyc. So they got used to going in the litter tray. They still go in the litter tray (we keep one in the bathroom for the 19 year old cat who is a bit doddery to go outside now). The kittens go outside all the time but come in to do their stinky poos.
How can I train them to go outside? It's like they always save an especially vile turd for visitor/house viewers/anyone.