I'd say she's trying to make that litterbox her's too, and show him who's boss. My cats don't care which one they use as long as it's clean.

As for the food, you might have to end up feeding them seperately. Mixing the food is never a good idea...he'll get what he doesn't need and so would she. That kitten food can pack the weight on too.

The shivering is worth a vet visit though. I don't know...maybe she was just happy your child was home, sometimes Toby purrs so much his body rumbles and could be mistaken for shivering. Good luck and good thoughts for the visit!