Should MS wrestlers ever use them NO.
Is is against MS wrestling rules? Well there isn't any actual MS governing body and rule set in Indiana, so most MS programs just adopt a modified version of the HS rules. So the issue of plastics being illegal would be unclear. As far as it being illegal for ISWA Middle School State that would specifically point to the rules that governing body, and USA Wrestling, has in place for all youth athletes.
But again should they use them to make cut weight NO. Should MS and Elementary wrestlers be cutting lots of weight in any way. Again NO. If a few days of better portion control (not starving) and hard wrestling workouts don't get them real close to weight then that's probably not the weight class they need to be wrestling. Sure they may have to run a little that morning to get off the last few tenths of a pound, but no to the extent of using plastics or any similar item to get there. Really this is about the same logic that should be applied to most HS age wrestlers too.
I get other kids may do it, and I also get that some of them end up being great wrestlers despite how they made weight, but if you really look at both side there are just as many stories of wrestlers not having success after a hard weight cut, or hating the sport due to constantly having to make weight using these type of methods. It's MS work on improving the technique, getting mat time, and learning from your mistakes. Get them educated early on healthy dieting, portion control, and self-restraint. Don't go down the road that will developing a negative attitude towards what it take to make weight class. Constantly cutting weight each year using extreme measures is one of the biggest negatives wrestlers will say about the sport, and I would guess a leading cause for "burn-out." And don't self create yet another excuse for the wrestler (like lack of energy from cutting) for why you weren't able to beat a tough opponent.