I'm fixing up my newly acquired M1123, converting it to a 4 man, and have two small kids (3yo and 6yo), so this is something I've spent a lot of time working on and trying to dial in.
Standard HMMWV Seatbelts (and every aftermarket/replacement retractable) are inertia reel and will not properly secure a person under 75lbs under all circumstances. A frontal 60mph impact and they'll probably lock, but a 30 mph bump might send them flying. I installed the standard seatbelt mounting hardware, but ordered this belt:
Check out the deal on 3 Point Seat Belt - Non-Retractable - Plastic Push Button Buckle - with Sleeve at Replacement Seat Belts | Lap Belts | 3 Point Seat Belts
www.seatbeltsplus.com
It's due here in a few days and I will report back for how the installation goes. When the kids are old enough, I can always swap back to the standard belts, but for the $80 it cost, to know that they're going to be secure when the doors are off, seems like a no-brainer.
If your kids are still in car seats with clips, you can always buy some D ring tiedowns, securely mount to the tub (grade 8 hardware, thick washers, etc.) and will likely be fine.
Good luck and I'll post up when I get them installed.