jackrose - my DD3 (who is almost 2yrs old) is going through the exact same thing. And like you, I know all the "normal" things to try with constipation. We eat lots of fibre, fruit and veggies. My oldest DD (6yrs) has the same thing. My second DD is just fine. Same diet, same lifestyle. But I will be trying the prune and apple juice mixture (they wouldn't drink the prune juice alone, but I didn't think to mix it.) and some probiotic yogurt and see where that gets us.
I totally understand your feelings watching your baby try to poo, and screaming in pain, and you can't fix it. It's a horrible feeling. They don't understand, and you can't really help. I hate it.
Here's hoping some of these suggestions work for these little ones!