My daughter had constipation and was afraid of pooping after that. I asked if it hurt. She said it did. I told her I had some dried plumbs that would help it feel better....and make the poop easier to push out.
My kids beg for dried plumbs (prunes) now - they call it "1-2-3" because I put one in each hand and one in their mouth. I usually have them drink a glass of water with it.
Even though it doesn't actually help until the next day, my daughter found it very reassuring and often asks for 1-2-3. (much to the astonishment of my own mother..hehe)
Try sitting her on the potty after you change the first poopy diaper. My son likes it when I watch for his poopy to fall into the toilet. "Did your body make more poopy? Oh, I think I see it. Can you push it out? hehe...it splashed. That was a good one!"
I know boys are different, but my youngest hates having the poop next to her bum. Often she'll wait for a clean diaper before she poops.