I sympathise with constipation relating to taking iron. I have a history of low ferritin levels, so have experience of taking iron and the lovely consequences!
I agree with everyone that Lactulose takes really sweet but not too horrible, and also wanted to say that I think it's a great thing to have in the cupboard, since (as far as I can understand it) it works by basically drawing water into your poo so that it softens, which means that it is effective but very gentle. Hope it works for you!