If you child has an issue on which table they are on, you need to talk with them about how they are coping with the work, is it too easy, too hard, or just right. It should be a mixture of everything, too easy helps with confidence, too hard to stretch them, but on the whole, they can just about cope with it.
This is what is going to benefit them the most. If a child is put with children who are working way ahead then they can feel thick, stupid, dump, lose confidence etc., then this isnt going to help
So instead of stressing about which set they are in, I think it is more important for children to realise they are working at a level that suits them and need to realise that all children are different.
Just as their are brilliant sports children and those who cant play sports, there are children who thrive at the academics and those that dont.
Teach your child that there are jobs for everyone to match their skill set and talk about all the different jobs in the world and why they suit different people with different abilities.
Make it positive and be positive with your children. Never be embarrassed or upset because your child isnt in the top set. It really doesnt matter