I have two sons that are young.  The oldest is starting to shoot his .22 rifle.  I am putting together a little starter collection of guns for both of them.  Just the basics like 22 rifles, 20 and 12 gauge shotguns, I have enough AR's to give them each one etc.  I don't personally have anything in the AR10 range.  What I would like to do is get 3 lowers or upper/lower sets that we can build.  I might build mine sooner rather than later just to have, but I would like to have theirs for when they are old enough to shoot them so that we could do a build with whatever the caliber of choice is at that time.  Any recommendations?  I haven't looked into big boy AR's in a while, but my recollection is that they aren't the same are their little brothers where everything is essentially universal fit unless you get into specialty stuff like piston kits or manufacture specific designs.  I remember looking at Mega Maten sets that had the DPMS and SR25 compatibility, but I was hoping someone could point me in the right direction for options to consider because things have likely changed since I looked at them

Aero M5

X2 on Aero for builder parts.
One further point, you can get matched receiver sets and also Cerakoted sets

Looks solid and price seems reasonable.  Any particular reason for recommending?

Just trying to be helpful. (Not a shill)
Ive built several small frame AR’s  out of their  m4E1 pattern.
And one M5 thats a tack driver.

Good enough endorsement for me

I bought 3 Aero M5 (not M5E1) receivers sets.  Happy thus far

Nice.  What did you build?

I have a 18" 6.5CM upper with Criterion barrel, a 16" .308 upper, and I'm putting together a 22" 6.5CM upper at some point.  I have 2 lowers built out, one with a Magpul PRS stock rifle buffer tube and another carbine buffer with Magpul STR.  The 3rd lower is still in the box just need to decide which stock setup to go with.  Currently waiting on optic upgrades for the 18" and 16" uppers but here are some pics