Colorado park reservation & underage drinking

I am planning on having my reception at a reserved pavilion in Estes Park (rented from the city). We will only have 2 guests under 21 other than small children. We are not planning on having a bartender, but just having beer & wine on ice. My concern is the 2 under-aged guests sneaking some drinks, with or without permission of their parents. Even with the permission of their parents, it is still illegal in Colorado because it is not on private property. In this case, I would technically be purchasing and supplying alcohol for minors and I would be liable. How can I prevent this without hiring a bartender, aside from telling their parents to watch them?