I served as Treasurer or as the accounting point person for over a dozen youth groups or official in-school teams over the years that my kids were in two large, well-off districts over 15 years. I would say cash skimming or theft/giving away of inventory took place in over half of them (of which we caught perhaps 50% of the individuals involved and managed to halt most of the rest.
To give you an idea of how pervasive theft can be, one of the school AD's and the head coach of one kid's club team were caught diverting cash (I was not involved with former, but found the hole in the books for the latter when I was drafted for the financial oversight role. We're talking thousands of dollars in each case. For that matter, one of the city officials in one of the communities stole nearly half a million dollars from cash skimming from the Parks department over a number of years.
I always made it a point to publish a monthly accounting of bank activity, receipts of for all purchases, sales figures, and weekly inventories of equipment, concession stocks, etc.- and required two signatures for ALL checks. It usually made gaps rather obvious to everyone involved - when you buy 2000 merchant discount cards, sell 1500, and have 100 in stock, most anyone can do the math.
I would not be surprised if some insurance companies no longer cover cash thefts from booster clubs or athletic departments.