New volunteers usually worry about the same thing: not knowing enough science. In practice, subject knowledge is rarely the limiting factor. The volunteers who make the biggest difference are the ones who manage attention well.
Ask before you explain
When a student is stuck, the instinct is to give the answer. A better first move is a question: what did you expect to happen? Students almost always have a theory, and working from their theory teaches more than replacing it.
Say when you do not know
Children ask unanswerable questions constantly. Guessing confidently undermines the whole exercise. Saying that you are not sure and proposing how you might find out models exactly the behavior the workshop is meant to teach.
Watch the quiet table
Groups that are struggling are often silent rather than loud. The noisy table is usually engaged. Checking on the quiet one first is a reliable heuristic.
Handle the mess early
Water activities spill. Setting expectations at the start, keeping towels within reach, and treating spills as routine keeps small accidents from derailing a session.
None of this requires expertise in hydrology. It requires patience and attention, which is why we can bring on volunteers with no science background and have them contributing within one session.