It's worth pointing out that a high pain tolerance doesn't reduce the damage done to a person. If a child is punched in the stomach the lack of protective muscle and flesh around their abdomen would probably result in severely broken ribs possibly resulting in fatal organ damage.

If they're damaged in a fashion that is non-physically impeding such as cuts to fleshy regions and the like it may allow them to continue with no effect, but otherwise a high pain tolerance is only useful with at least some decent defense. 

Though I don't think a spellcaster with a weak physical body is an inherently bad idea, as my character Solomon is similar (though incredibly old, rather than young.)

Having a demon possess a child is fairly cliche, but it does negate the main limitations of an ordinary child's lack of intelligence and realistic combat experience.

Also Impaqt, try not to be a dick, it's particularly nonconstructive in an advice thread.