Honestly, just stop cleaning the brushes. Leave them covered in soot and creosote in a corner of the garage. Kids hate getting filthy, and that black grime is a natural deterrent. They touched mine once, got their hands and clothes all messed up, and never went near them again. It's not pretty, but it turns your tools into the last thing they'd want to play with. A clean brush looks like a fun hiding stick, but a dirty one is just a gross chore.
Ever tried just locking the brushes in a high closet? We started keeping ours on a top shelf, and the hiding game stopped cold. You'd be surprised how a simple out-of-reach spot fixes things.