Here’s an alternate to a (probably expensive) and apparently dangerous warm up toy:
Sew dry field corn ( you can get at feed or nature stores) into a small pillow-style bag (1 ft. square is a nice size) Fill it about 1/2 to 3/4 full. You can microwave these bags for about 60-90 seconds, depending on the wattage of your mic, and they will safely retain heat for upwards of an hour.
My mother made one over the holidays for my 6 year old daughter and now she won’t hardly go to bed without it. Nice and toasty and doesn’t explode.