[quote=@Bartimaeus]
If she has magic and is being assisted by a physcological demon, it is surely possible.
[/quote]

Why not just take this WHOLE concept, make it a golem, empty puppet, whatever; have the creator summon a demon into it - demon is too strong, gains control of golem body, still looks like a kid. You can make it look like a fairly realistic child(if the creator was summoning in the lost soul of his recently deceased daughter, and said demon decided to take over).