This is ridiculous of course.
But for the family of four, simply check out a couple of pound shops; you can now find hi vis vests in quite a few of them. All one size, not great quality, but who cares - for £4 you can throw them in the back of the aircraft and be legal and worry about more important things.
I had a nice little sideline buying these and reselling them on Ebay for a while, but full time instructors don't have time for that kind of stuff in a summer like we're having.