Just puting my two cents in here.
I used an NPC child vampire in a game I ran a few years ago to great effect. The NPC ended up being the characters greatest ally. Physically she was 10 or so but mentaly she was an adult, she hated it (being forever 10 is rather horrible) but diveleped a bit of a shortman complex. and through ruthless use of blood bonding actualy rose to the position of Harpy in the city and controled (not owned her ghoul owned it) a popular night club. I came up with the NPC on the fly, I didn't want some Cliché vamp club owner and I figured it's a horror game why not show the players there not in Kansas anymore.
it worked and the characters (all fresh neonates) felt sorry for her, and later became her friends and many side arcs centered around the child vamp from then on. Hell on request when one of the players lost a character the NPC became the sire of his new one (Spawning even more stories.)
in my opinion a child vampire is a horrible curse, obviously I had her continue to grow mentaly so she was a viable character in the capacity I needed. Even so traped forever in a child's body would put a severe handicap on anyone; supernatural power or not. so the said vamp must "Grow up" fast and get creative with what they have, namely the Blood. if a child vamp was traped forever with the same childlike outlook the situation is even worse and they are a breach wating to happen. they would quickly lose there minds and sucome to the beast... I mean children don't have much in the way of self control.