Seems your own judgment was that they deserved a tip.
Having lead tours I know that tips are appreciated - the amount need not be a percentage but should be somehow related to the experience. For a short trip around a museum enough for a coffee seems about right to me. Perhaps with extra for a doughnut if they were really good
It's the feeling appreaciated that really counts. That said, we were pretty poorly paid and tips made a big difference to savings at the end of a season. Even if that's not so true of volunteers I'm sure it'll put a smile on a face