Oxymoron - a combination of contradictory or incongruous words.
Seriously, it's difficult to make anyone excited about volunteering for specific tasks. What you can do for sure is make folks excited about the event, what it means to the kids and the community or what great things can be purchased with the fundraising proceeds. If folks are excited about and supportive of the event, they will be more likely to pitch in where needed.
If you can't excite them, you can at least make it bearable by keeping a few things in mind:
~ Clearly define necessary tasks. Nobody likes to fail because they had to guess what was expected of them.
~ Loosely define arbitrary tasks. If it's just for fun to enhance the program, give the volunteer lots of freedom to make it fun or interesting.
~ Keep work shifts short. If the event is three hours long, try to keep individual shifts to 30 minutes. Running a cake walk or ring toss is fun for 30 minutes. Longer than that and you'll see grown ups cry!
~Provide necessary supplies or a list and reimbursement instructions. If you ask someone to teach an art project, they'll expect to show up and have everything they need there. If they need to 'develop and teach' the project, let them know the details way in advance.
~Basically, respect the volunteer, their time, budget and abilities.
~Thank them profusely afterwards!