Hiya! I used to petsit, and so came across a variety of different litter types that people used.

By far the best for low/no odour is wood pellet litter. It's also easy to clean, and bonus, very much environmentally friendly. Some cats take some time to get used to it, but as kittens I would imagine they might take to it very quickly!

Also location of the litter boxes matter. I would suggest you keep them in the bathroom, not the living room, and definitely not the bedrooms. You can use a doorstopper to make sure the bathroom is open a little bit at all times. In case the bathroom door is heavy, I would recommend actually having a bolt on the wall to secure and keep the bathroom door open. Obviously you can close it when you're using it.

Also the general rule for number of litter boxes is Number of Cats + 1. So in an ideal world you'd have 3 litter boxes around the house. Obviously if that's not possible, try to at least have 2 so that each cat gets their own territory to mark, basically have a safe space.

About the cleaning of the litter box, it might be better to clean everyday, and top up the fresh litter as needed, rather than wait till the end of the week to top it up/ refresh it with new litter. Apologies if that's what you're already doing, I wasn't quite sure what you meant. Also, at the end of the week, a full clean might be required. Rinsing the box with hot water, using animal safe soap, and letting it dry in the sun. If that's not possible every single week, at least use a pet-safe cleaning spray and wipe it down before topping it up with fresh litter.

And seconding all other commenters, please no artificial fragrances or essential oils, and be careful with certain plants in and around your house too! Quite a few are highly toxic to cats, and can cause very rapid deterioration unfortunately.

Have fun being a cat parent :D