We've been struggling with this since before he was a year old. He got in a cycle where it hurt, so he kept it in, it got harder, so it hurt more, kept it in, feared going, etc. To this day, at 2.5, he still has a hard time. We tried everything -- he would only eat whole grains and lots of fruit (especially prunes) and would still have trouble. For a while it was only meat and fruit, and still had the problem. Finally, gave into doctor's recommendation of a tablespoon of miralax in his food/water/drink per day. This helped a ton when we first started, around age 18 months I think. It continues to help now, but we still need lots of prunes and fiber to keep him regular. I think he's finally old enough to understand when I tell him it only gets harder if he fights it, and he has to let it out.
Anyway, he never went for prune juice, but that works for some, and another recommendation I heard is chia seeds. You can mix them into a smoothie or something to get them in. These are supposed to be great. Also, DS won't eat a dried prune, but he'll eat the baby food pureed ones, so I try to give him that regularly. I have not had to resort to a suppository except maybe once in the past year. There have been times, though, that it has been hard for him to go, and I believe it's because he didn't have his full dose of Miralax. Miralax is supposed to be safe and have no long term effects according to doc.