I found a good price on a RRA receiver.
Now I need to decide what to build on it.
I would like to build it with my 11 yr old son and have him be able to shoot it.
How do I decide what brand of parts to use and what type of AR configeration to build.

go thru the pictures other other peoples build and see what you like

Mike

build a 16" midlength, excellent first rifle. Why not build with RRA parts the fit and finish will be a perfect match to the lower. I recommend ADCO

If your looking for an affordable kit go with M&A parts.    +1 on the midlength.

www.m-aparts.com

also, collapsible stock would be a good idea so it fits both of ya

+1     for ADCO

I also have a boy but he's only 10 y/o now so I'm still planning on what to build for him in about a year or two from now.

I'm thinking of getting a 16" lightweight CAR flat top upper or same with a mid length Gov't profile barrel.

The standard CAR handguards, 5/ 6 position CAR (not M4 type) collapsible stock and an A1 type grip would be a good choice and should fit a small person.

If my son thinks that the rifle have too much power I could build him a dedicated .22 LR AR upper so he could practice with it and then when he's ready move up to the 5.56 caliber upper.

As for brands just avoid the known bad brands and just go for the best bang for the buck.

In my own case I'll probably get parts from CMT, Stag Arms, RRA, Double Star, Cavalry Arms (furniture), Bravo Company, etc.