But I keep coming across the idea to just use an 18% grey backdrop
NOOOOOOOOO! It won’t work, you’ll have to flood the background with light to get it to go white and unless you nail them down, kids will keep running into the bright zone making the effect you’re after impossible.
One thing to watch out for is that Vinyl is REALLY heavy and you won’t be able to support it on cheap light stands plus it creases badly if you don’t look after it (guilty!) and you can never get it flat again.
But with a little effort I tracked down a local ish company that repairs bouncy castles and got a piece of WHITE vinyl 9tf by 15ft at about half the cost of buying it from a photo dealer.