I understand completely what you're going through. My 2yr old daughter has been suffering from constipation really badly for months now. She'll go 3-5 days between poos and gets so grumpy! When she does finally poo it is so hard and she's basically screaming as she passes it. It's so bloody awful to watch and know that I can do nothing to help her.
She had an endoscopy about 3 weeks ago to try and find out the reason she vomits (a WHOLE different issue!) and I discussed this with her gastroenterologist who suggested giving her parachoc to soften the poo so it wouldn't hurt to pass it. It didn't seem to make much of a difference at all but I was talking to our peadiatrician about this today and she suggested giving her senokot aswell as the parachoc. Apparently its in a powder form and start off giving her a quarter of a teaspoon mixed with milk, weetbix, or whatever she'll eat and adjust accordingly from there. The parachoc is supposed to soften the poo but the senokot will actually work as a bowel stimulant to help get the poo out!
Hope this helps, its an awful thing to watch!
Jen, Qld, 06/03 girl & 04/05 boy