I got my cat spayed when her kittens were 7-8 weeks old. The vet said providing they were eating solids properly they no longer needed mums milk. I was instructed to seperate kittens from mum for a clear week to allow her milk to dry up.
This may sound odd, but I have a big shower, so I put the kittens inside the shower, with food, water, bed and scratching tower. This way their mum could come and see them, check on them, but they couldn't feed from her.