Learn English – What word should I use to describe a kid who has pooped in their underwear

A callithumpian

1836, U.S. colloquial, probably a fanciful construction at one time designating a society of social reformers, then in reference to "noisy disturbers of elections and meetings," and most commonly "a band of discordant instruments."

Actually paired adjectivally with chaos in stanza 84 of David Van Alstyne's 296-versed mouthful of a poem:

buccaneerishly galumphing into grievous garboils of chthonian uproar and terpsicoresan bedlam, and for gallivanting in great gyres of callithumpian chaos in a three-ring circus of near-simian agitation

The word I use at work is just "late". They're all late.

Unfinisher doesn't exist, but non-finisher does.

Your question is a little ambiguous though. Are you looking for a word for people who do finish, but after the deadline (flaky), or for people who never complete the work at all (non-finisher)?