Do kids lose fat and gain muscle at puberty?
The body often stores up fat to be used for energy just before a child goes through a growth spurt. Basically, you get wider before you get taller. For boys, their testosterone levels can increase after puberty, so yes, they have the ability to build more muscle after puberty then before. Girls will often gain a little more weight and get a little more body fat in their feminine areas - chest, buttocks, hips, after puberty. The body is preparing itself to be able to carry a baby.
Hope that helps!