I would say the most important thing to watch is the heat, depending on where you're at of course. If you can, freeze large blocks of ice (for example put a couple cups of water in a sandwich bag and freeze) and keep in the cooler, and place these under and/or next to the cage, or even in the cage, to help keep the ambient temperature down. Keep them in a shady location with a light breeze. Make certain they don't get chilled, of course. Make certain all the cage doors are escape-proof, esp. for the hamster, because if anyone gets out in an unfamiliar location and becomes frightened, it will be difficult for them to find their way back. Bring plenty of extra water and change their bottles frequently as another way to keep them cool and well-hydrated.
