Cats are smart, they will know. It is when they are little that you need to worry.
9 weeks is a bit young to be away from their mother. Where are you getting them from? They shouldn't leave momma until 12 weeks old roughly. They are missing out on a lot of things by leaving to early. The mother teaches them so much, litterbox, socialization skills, manners, cat behaviors, etc. Unless they are orphans of course.
Himalayans are great cats! I have one and my mom has one. Mine was dumped at a car wash and I adopted him at the Humane Society. He is a WONDERFUL cat. Never had a better, well behaved, friendly cat. Please post pictures if you have them! What color Himmies are they?