Well, cold processed soap doesn’t need a stove. The lye reaction creates the heat. But, lye is a caustic chemical and this type of soapmaking has a lot of steps and requires multiple ingredients. Obviously you’d need to be very careful with the lye with children.
Another options would be to get some soapmaking materials from a craft store like Michael’s or Hobby Lobby. You aren’t making the soap totally from scratch, it’s just melt and pour in a little crockpot type appliance or microwave.
So, do you want it to be more of a science project with precise measurement, chemical reaction, stirring, pouring, etc… or do you want it to be more of a fun craft project to go along with the book?
That, to me, would determine which way to go.
Go to teachsoap.com/ and look around their website to decide how you want to proceed. They discuss melt-and-pour as well as cold processed, and have recipes for some simple things kids can do like bath fizzies.