I have player who wants to play a child (10 year old) this is the child from a previous character of theirs how would you suggest handling this mechanically lowered brawn, ST, WT. etc.

I wouldn't make any mechanical changes. If the player wants to play a child he should stat the character appropriately, which is doable in the existing system.

Keep brawn at 2 but as a "movie" themed game you can get away with doing other crazy things, like high int/mechanics or high Agi/piloting. And talents aren't age specific.

Here

http://community.fantasyflightgames.com/index.php?/topic/120741-child-pc/