Eeesh. Giving Kumon the benefit of the doubt here, maybe they got their wires crossed and presumed you knew it was a trial shift. It's easy to say "why didn't you say anything at the time?" but when you're in that situation you don't think to be like, "er, excuse me!? What's the dealio here?" you just get on with it, so don't feel like a mug.
If you are interested in the job, call up and tell them you enjoyed spending time tutoring the children and wanted to get in contact to see what the next steps are. Also, you're well within your right to ask to be compensated for your time.
If you're not interested in the job, call up and explain that you were confused with the initial so called interview and ask to be compensated for the 3 hours you were there tutoring.
From personal experience (working a trial shift at a pub), these people are probably more than happy not to pay you but will be more than happy to pay if you bring it up with them. I feel like they were short staffed and tried to pull a fast one!