Help me build this Grendel!!! On the cheap!

[LRRPF52]

Here's the updated Build List, with parts you have in blue.

Lower Receiver Group
[COLOR="blue"]Lower receiver...check
Grip...check (assuming it will be good for 5yr old's hands)[/COLOR]

LPK
Take Down Pin & Pivot Pin
Carbine Extension Tube
Trigger Guard
Bolt Catch
Safety Detent & Spring
Mag release button
Buffer
Extension Tube Lock Nut (Castle Nut)
Receiver End Plate
Recoil Spring
Buttstock suitable for 5yr old, as well as adults

Upper Receiver Group
Upper Receiver
Charge Handle
Bolt carrier
Cam Pin
Firing Pin
FPRP
6.5 Grendel Bolt
6.5 Grendel Barrel
[COLOR="blue"]Ejection Port Cover
Ejection Port Cover rod & C clip
Ejection Port Cover spring[/COLOR]

Handguard w/corresponding barrel nut
Gas block
Gas tube
Muzzle device

Sighting System
Optic
Mount