One of my kids had this problem. Its called encoporesis. What happens
is that at some point, for whatever reason, a kid starts holding it in.
Could be any reason - painful constipation, fear of using the toilet
away from home (lots of kids don't want to use school bathrooms),
desire to keep playing instead of use the bathroom, whatever.

Then what happens is a pernicious spiral. Kid resists going, and
ultimately the kid's colon gets stretched out, and they lose normal
sensation and function. In other words, they can be packed with stool,
but they can't tell, they don't get normal sensation, and they never
properly empty their bowel.

A frequent hallmark is frequent small poops in their pants - because
what is happening is that their bowel is so full, as more stool comes
down the pipe, it has nowhere to go, and it starts pushing small
amounts out involuntarily.

When my son was first examined by the doc, it was clear this kid was
majorly backed up.
What we had to do is try out various doses of laxitives - we started
with mineral oil, which wasn't enough - we had to go to Ducolax. The
doctor's instructions were to play with the dose until we had him
having two good movements a day (i.e. big ones). The point was to get
his colon empty, and keep it as empty as possible, so it could shrink
back and normal sensation and function would return. This took several
months, at which point, we gradually stopped the laxatives, but kept a
close eye on his regularity to make sure he wasn't sliding back into
the pattern.

Its way more common than you think, particularly in boys for some
reason. Its been several years since this happened, and its never
recurred. I wish I'd known about it from the get-go, since like you, I
was hugely frustrated, and just couldn't understand why an intelligent
kid kept pooping in his underwear! The answer....he really couldn't
help it.

Mary G.