Don’t build it for the kids buid it for yourself. With CW2 on the way you never know when you might need it.
I suggest reinforced concrete and a ventilation system that ejects tear gas quickly.
Basement corner.
Row of cinder blocks surrounding with openings for shooting lanes; no more than 2. Mortar not required.
Second row of mortared cinder blocks, 3 feet away from inner row, with corresponding shooting lanes.
Industrial-duty air-movers and half face mask, multi (gray) canisters.
Full supply of ammo, sabot-slugs preferable if you have a shotgun.
In corner directly opposite, one dozen remotely triggered martial-arts style “stars”.
Come get me.