For the second time I've helped at our local museum with groups of visiting children. I like doing it, and the (paid) organiser is very efficient (which often isn't the case with voluntary work). But I think the organiser should send an email to say thank you, it wouldn't take her a minute and, if someone is giving you their time for free, you should show some appreciation. AIBU? This isn't a full-blown annoyance, just a niggle that, if they want people to help, they should be more polite.