I know exactly what you are going through,we have had the same with Hannah basically since birth. I know consider myself to be a bit of an expert LOL!

I have had her at the Dr and the paed at different times and some things they said that might help are
- Lots of water, fibre without fluid means big hard poos
- milk can increase constipation so maybe try watering it down or switching to other forms of dairy (apparently it is only milk thats an issue)
- Child laxatives like Parachoc are ok to give long term, they dont act like adult ones so the bowel wont get lazy
- when things get really bad like day 4 or 5 then maybe a suppositry might help

We have a bit of a rule that one day without a poo then she gets parachoc and kiwifruit and we play games like washing the dishes where she will drink lots of water (dont ask!) because with Hannah we have found it esculates quickly to the point where she will be scared of doing poos coz it hurts. I keep the parachoc going for a few days coz or even a week at lower doese until I think things might be back on track..

Anyway I hope this helps, its a bit of a learning curve. Who would have thought poos would be so involved!