Probably sound like I'm advertising here but just wanted to share because it's been so revolutionary for my children.
DD has suffered with constipation from about 15 months or so. Got the usual advice from doc / HV of that I needed to improve diet, more veg etc. But nothing worked and she ended up taking movicol for a while.
DS started with it and I couldn't cope with it all starting again! So decided to take action. It was vaguely controllable if they had a very high fibre diet with loads and loads of fruit and veg then they would be ok, but it was like we were having to compensate for something that was making them constipated.
I convinced myself that is was drinking cows milk that was causing it, with the timelines it made sense. So I was going to put them on soya milk and try that.
DH found this A2 milk in the supermarket which is supposed to be easier for them/us to digest so thought we would try that first. It has a different protein (A2) to commercial cows milk (which has A1) which is the same as in all other mammals milk.. it's just commercial cows milk that is different (I.e. From the black and white milk cows). So I think the A2 is from jersey cows or similar, but still normal milk.
Anyway, it didn't happen immediately, took 3/4 weeks to kick in but now their stools have completely changed in consistency, not at all firm and easy to pass at least daily.
I think it is definitely this that has made the change. I just wanted to share in case anyone else was having constipation issues with their kid!