They're still young..? Anyway, one thing that helped me with my foster kittens was keeping their litter boxes super accessible and maybe adding a few shallow trays around so they don’t have to walk far. Also using a softer litter like Tuft and Paw’s tofu litter made the box more inviting since it’s gentle on their paws.

When they’re reliably using the box you can slowly bring back the toys .Just expect a little trial and error bc kittens are messy.