put a nerf belt feeder inside to shoot nerf darts.
[video=youtube;bzWHUruJnWk]https://www.youtube.com/watch?v=bzWHUruJnWk[/video]
I'da just made it a aircooled .30 or .50 BMG. Just a little more modern. Somethin' kids could more relate to.
That is awesome. I actually had the same idea, to make one for my (future) kids. I would further suggest "linked ammo" by just stitching empty casings (or sections of wood dowel) into a strip of fabric. The old doughboy guns had fabric belts, IIRC, so it would be period-correct!
Sarco has .30 caliber M1919 cloth belts. Load it up with dummies and it would make a great addition.
Browning Cloth Ammo Belt, 100 RD
Heck I could have fun playing with that toy gun. I had a cap gun that used roll caps, it automatically fed the next cap and fired with each pull of the trigger. Wouldn't have been difficult to stick the guts of it inside of your creation.
Or paint them.