You would need to license on one of the UHF itinerant frequencies. This is pretty easy to do, but can be will cost more than the GMRS license. Unless you are in a seriously urban area, you shouldn't be an issue on GMRS. Keep your power low, you won't need much to cover a few acres. Make sure you set CTCSS or DCS tones on the radios.
Using an itinerant frequency is no guarantee that you won't get interference, or cause interference. For what you are doing GMRS is a better choice. $85 for 5 years, covers you and your family.
SP50 isn't a bad radio for that sort of stuff. Easy to get accessories for, and small enough to fit in a pocket.
Thanks for trying to be legal!