Instead of using an electric heating pad with dry heat, consider making your own heating pad with plain white rice and a white cotton sock.
First, you will need around 3 cups or so of plain white rice (NOTE: though I don't buy white rice to eat, I do use white rice to make my rice sock heating pad. A gigantic bag of white rice is very affordable. One bag will last you a year.)
Then, find a clean white cotton sock. I would suggest washing it well with a little soap and bleach, rinse it very very well. Another thing I would suggest is that you start saving all your odd mismatched cotton socks for such things as this in the future.
Fill your sock with between 2 to 3 cups of rice, according to your preference. Place it on a clean microwavable plate and heat it in a microwave on high for one minute. After one minute, take it out and turn it over and heat it another minute on high. A total of 2 minutes should be sufficient. If you want it warmer, you can add a few seconds at a time.
Wrap your rice sock heating pad in either a clean white cotton pillow case, towel or a recycled white t-shirt. This rice sock will stay warm for a very long time. It also produces a moist heat, instead of a harmful dry heat. It is safe enough to use with children. NOTE: If you use it for children, be careful not to make it too hot. Also, quickly knead the heated rice sock in your hands to evenly spread the heat before giving the sock to a child to use. This precaution will eliminate the danger of a child being burned by hot spots that might be hidden in one or more areas of the sock.
You can use the same sock indefinately. I use mine until the moisture is just about gone. After heating it up the first time, you will notice a smell simular to popcorn. As the moisture is taken out of the rice it will more and more resemble the smell of popcorn ... when it starts smelling more like burnt popcorn, it is time for a new rice sock.