@SarahGen-- I've used instant hot packs before and I never had this problem. I don't think it's a danger if it's handled properly. The plastic is very thick and it won't be damaged easily. But you shouldn't allow kids to use them by themselves.
You could get hot gel packs but those are not instant. They have to be boiled or warmed up in the microwave. Instant hot packs all contain chemicals. Lots of people use them without any issues.