J.F.
Hi N.,
This isn't about being "silly," but rather, it is an issue of financial integrity and providing documentation for every dollar, which is required if your organization is a non-profit. (ETA: This is a completely different situation than the first response below where it was a personal event hosted by parents. In that case, a person is free to tip however he/she chooses. Your situation isn't about proper tipping etiquette. It's about following established rules for how non-profits handle financial transactions. It's a very important distinction).
If this is for a PTA or other similar non-profit, for example, you can't just "give" them a cash tip. Nothing is ever paid in cash. [For PTA, this mandate is found in the "Money Matters" guide]. Every expenditure must have an invoice, bill of sale, check request, or reimbursement request, etc. and must be paid by check. If you are having your financial records properly audited each fiscal year, as is the requirement for every non-profit, this would be something that would be flagged by the auditor.
Our PTA does not tip. We get our own volunteers and high-school and middle-school kids who need service hours to staff our events.
When we do use outside service vendors (which happens several times a year), we discuss this policy when we are entering into contracts with them, and we tell them we do not pay cash tips. We make it clear up front. We have strong relationships that we've developed with our vendors over the years, and they know that we are a non-profit that services children. So, we agree on a fair price so that both sides are happy. Year after year, our vendors always want to come back and do our events.
Some businesses (like the lighting company we use for our Harvest Festival, for example) do include a set-up and delivery fee built into the contract that covers the extra time their staff have to spend, and that's fine, because it's on the original invoice. It's documented. It's a legitimate expenditure.
I would suggest you talk with your board, particularly your president and treasurer and also become familiar with the financial rules that govern your organization. Some of the biggest problems non-profits get into are the ones that involve finances and people not knowing or understanding those rules.
Good luck with your event!
J. F.