My wife and I are ATVers. We joined an ATV club recently and some of the group rides have quite a few participants. Having a low-powered repeater mid-point would help ensure that the front and rear riders, as well as everybody in between, hear each other.
Well, as you know each family will need a GMRS license if the want to be legal.
Also, most of the cheap consumer grade GMRS radios will not do repeaters.
I used GMRS with family on ATV rides, and it worked fairly well, at least until I was able to get them to study for their ham license and switch to 2 meters.
A couple of things to consider:
-Some ATV's have noisy ignition/electrical systems and will cause issues. If you are going to power the repeater off one of the ATV's, consider some filtering on the power connection.
-Repeaters are going to have duplexers in them, and they will not like being beat around on a rough trail. The duplexer may go out of tune quickly and make your repeater pretty useless. Retuning the duplexer after each ride is going to get expensive if you don't have the correct test equipment to do it.
-If everyone is covered by a GMRS license, you'd do better to have them get better radios and antennas.
We used to use Motorola HT-600's and P-200's on GMRS, they stood up to the abuse pretty well. Better antennas than the consumer stuff.
If anyone is running UTV's, they can add a mobile radio and a good 1/2 wave antenna on the roll cage and repeat traffic if needed. It takes a bit more work, but mounting a small mobile and a decent antenna on a traditional ATV is also a good option.
I'd really be concerned about the duplexer in any repeater in an off road application.