Congrats on your kitties. lol litterbox
you could try a different litter as some are pleasanter to humans than others. But if you do change it, use a little at first mixed in to replace the old litter slowly so that the cat gets used to the new litter.
Yeah you shouldn't let kittens outside at all,
until they have been neutered if possible and at least until they have had their vaccinations, so the age is important. At any age, when you get a new cat they should be kept in for at least 2-5 weeks to bond with you before letting them out.
I'm from the UK where in some areas it is ok to let a cat go outdoors if you have a safe outdoors environment (mine are indoor/outdoor). But the same dangers apply anywhere when you are thinking about letting a cat outside which you should consider; are you near a road, is your garden enclosed, is your garden kitty safe (no open ponds, rubbish etc), have your neighbours got pets and are there issues if your cats go on their property, wildlife dangers, known pet abuse in the neighbourhood? i'm sure other posters will think of others. You shouldn't let them out if its not as safe as possible. It is possible to keep them indoors if you create a stimulating environment for them.
That aside, first get your cat neutered (if hes not he'll stray). To get them to return you need to train them indoors first, to come when called. This is very easily done by calling them when its dinnertime by their name and by using their name to call them for treats. This is so they will return when called
When you let them out, always do it before dinner so that they are hungry and will return. The first few visits out should be for short times and supervised so that you can see where they go and how they react, you will probably need a friend or leashes to do this with two cats. You can judge when they are safe to be on their own.
ALWAYS bring your cat in overnight.
Good luck with your kitties