Most kittens learn the litter box very quickly as you described. They will still sleep there and play there, but less as they mature. At least they know what it's for!
You can go ahead and schedule Simba's spay now. I know it will be hard for you to keep her from eating, but just do your best.
There's nothing special you need to do to prepare the kittens for re-homing, but they should be eating solids well and be about 8 weeks old. Just tell the people taking them to feed them what you have been feeding them initially.
It sounds like things are going great!
You can go ahead and schedule Simba's spay now. I know it will be hard for you to keep her from eating, but just do your best.
There's nothing special you need to do to prepare the kittens for re-homing, but they should be eating solids well and be about 8 weeks old. Just tell the people taking them to feed them what you have been feeding them initially.
It sounds like things are going great!