I've used epipens on myself before, and had other people use them on me. I hate to tell you this, but it's probably the most painful thing I've ever experienced. That being said, the thing that motivated me do it was that without the injection, I might well have died...
I would always, always inject if in any doubt. I use my pen as soon as I have tingling in my face or lips, redness and swelling, and breathing difficulties. I was told it's always better to inject sooner rather than later, and if you think you should be injecting, then do; considering you will be getting medical help anyway, my doctor said that injecting it if you didn't really need it wouldn't do you any harm, but not injecting it could be a very bad idea.
In terms of resisting/squirming, yes I suspect the child would, especially if they aren't too ill by that time. I certainly find it hard to be still- I now tell people when I'm training them in how to inject me to make sure they hold my leg firmly, so I can't inadvertently pull away. The best way I can describe it is like when you get washing powder into a cut- it stings and burns. The pens themselves inject with a fair bit of force too, and often leave a bruise. The injection site is sore for me for a few days afterwards.
This all sounds terribly scary, and I'm really not trying to put the wind up you. The thing to remember is that at the time I need it, handling the injection is in many ways the least of my worries, and actually gives me confidence that this will make things better. Hope this helps!