Does depression affect a child's IQ score?

Yes. But only temporarily. When a person is depressed they very often don't really care much about the outside world. So, they may seem "dull" or "slow." but that goes away as the depression gets better.
It can. Depression has a way of effecting everything in our lives. Depression can slow down processing speeds and impair memory. Severe depression in the elderly has been known to appear dementia-like. Please bring your child to a psychologist or psychiatrist that specializes in children.