I think elementary aged children usually understand about skipping. I have seen recommendations to let them try it even if they are hesitant because it can always be reversed; if they are solidly against it, though, it doesn't seem a good idea.

My daughter was excited about skipping and it has worked well. We're pleased with the decision. Additional skips have been recommended, though, and she doesn't want to do that at present. She does go to a higher grade for individual subjects and does individual work, though, so that helps.