So here is the end result of this thread. A great big thanks to everyone who has helped me along the way with bits, pieces and advice.
Small avanti aggressor 26' frame
Reba dual airs, beautifully plush for a lighter rider (thanks Summit!) Steerer tube left long to accommodate growth.
165mm XT cranks with a 28t widgit and an 11-36 XT cassette. Perfect range for a 10yr old. AEST brand alloy pedals from ebay. Very light and spin beautifully.
Wheels have DT hubs, mix of rims, run beautifully (thanks Moorey, I put tubes in them out of respect for you)
Geax Saguaro's I had lying around.
X9 shifter and derailleur came off Dad's bike, just waiting a replacement jockey wheel.
Avid brakes also off Dad's bike. 180 front and 160 rear seems overkill for a kid, was planning 140 rear, but the frame is post mount and will take no smaller than 140. I am getting over the guilt of putting Avid's on his bike, but it means I got XTs for mine. Mmmmmm, very nice.
Cut off seatpost and short stem, narrow bars puts everything in place for a tall ten year old.
Overall, a cracking little bike for a very lucky and happy boy ( not shown in picture), put together for well less than $400, depending on how you calculate the value of bits taken from my bike.
Thanks again everyone!!