I used something like that many years ago, but no longer do. Still use various snaps & swivels, but only with other tackle, not fly fishing. Agree with what the others have said, they'll work well with some fly types, but not so well with others, and there are strength limits to each size.
If you're primarily fishing for panfish or stocked trout, particularly with kids, and they help, & fish are being caught, then use them.
Sounds like you're on the right track with teaching the kids how to tie proper knots, and that's important too. Just don't allow the snaps to be a crutch, and a substitution for a properly tied knot.