How does cfs affect children?

Variously. Children are not a common population subtype to develop chronic fatigue syndrome, but I assume they are prone just as adults. The underlying cause must be sought out and treated accordingly. The way it affects children varies, but physical symptoms - fatigue, weakness, malaise, somnolence may be present. Mental health - depression, slowiness, academic worsening may all be present.
Usually unaffected. If you mean chronic fatigue syndrome, young children are unaffected. Teen agers may sometime mimiccfs after a bout of mono.
CFS??? Please rephrase your question since cfs is not a typical abbreviation. CF is an abbreviation for cystic fibrosis.