May I suggest you see a gastro doc for children? My son was 5 when he still had this issue. What the doc said was at one point my son had a really painful bowel movement and it scarred him. So therefore, instead of going, they hold it in, which causes a backup in there. Then they have a really large mass, the bowels enlarge and they have leakage...and don't even realize they are going.
Our gastro guy had us do a full cleanout - that included miralax, an enema and that stuff you take for a colonoscopy (magnesium citrate?) and that had my son going better. We still have to give him almost a capful of miralax a day and watch his potty habits to make sure his poops aren't hard. The gastro guy will explain it all.
We were hoping our son would grow out of this. We are unfortunately still dealing with it at age 7 but waited until 5 until someone finally recommended we see a gastro doc for children.
We did have him a little too regular at first but we got the miralax dose finally figured out and it's doing him well.
I saw a suggestion below for fiber gummies. They are made by pedilax I believe and they are wonderful. We get ours at Target. Everyone has one a day - adults included and they help a lot. My son takes the fiber gummy and miralax still.
Good luck. I completely understand your frustration.