How do you get your child to use the bathroom?

I think the best thing and first thing you should do is visit your daughters pediatrician so her stomach ache can be addressed professionally. I think worrying about pooping on herself is secondary to the fact that she says she's hurting when she poops. There maybe something more to it than that.

Make sure that she is getting the appropriate fruits and vegetables, (5 servings a day) if this is the case, then take her to the DR. so that they can determine what is wrong, if anything. If nothing is wrong I would do postive rewards for accident free days. Please rule out a medical problem though before using any reward or punishment. There may be a reason and the child can't help it. Good luck with it.

If she is saying that it hurts when she poops, I'd take her to see a doctor. Maybe she has a physical problem that needs attention. At the very least, the doctor should be able to give you some ideas on how to deal with this problem.