Volunteer Bartender

We'll be serving beer and wine for our reception, the beer will be in keg form and in bottle form that'll need to be poured into cups (no glass allowed in Hall). My aunt and uncle have volunteered to oversee the bartending for the reception, they've both had a lot of experience professionally bartending and seem excited to help. Since I won't be paying for the services of a bartender, should I give them a cash tip or thank you gift? Should there be a tip jar that would allow them to earn tips? This is a lower budget wedding.