I alone have 3 zappers and even I have pain in the heart area after zapping. Claims that it is a pain from a die-off is just a bull****. Children are also much more sensitive to electricity, so better don't use zappers to children at all!
Honestly, there are other, much better allopatic solutions for children (and adults too) - like homeopathy. This is something I can really recommend. The "drawback" is, that you need REALLY good homeopathic doctor, you can not do it alone. And finding someone, who understand and feel the homeopathic method deeply, and is not just a pretending charlatan, isn't easy at all.
If you are entirely in the DIY approach, try Su-Jok and/or EFT. Much more gentle, no need of any device, but you need to spend some time to learn, understand and practice the techniques.
I know it is a Zapper Forum and a lot of advocates around, but Zappers are really experimental devices without clearly proven effects and questionable side effects. The Zappers have of course some effect, unfortunately no one understand how and why they work or don't work.