Normally, they should have just been thankful to be on the track, but since it became one of the best-known songs of the modern era, and it made some SERIOUS bank, they should each be entitiled to a small slice of the ridiculous profits made. It's just in good taste to do so.
Honestly, when the song blew up I'm amazed the parents of the kids didn't try to get some cash out of it. It's the fault of the kids' parents, as well as the school that they never signed any sort of contractual agreement to at least detail that they in fact sang on the track.
Also, it's not like Pink Floyd was a small band at the time, they were huge. Just being remotely involved with anything to do with them should have at least warranted a contract or written agreement through the record company.