You probably wouldn't get that far anyway. In order to remain within
the law, most GMRS/FRS radios can't use external antennas in
order to make the 500 mW erp rule. So, if you reduced power to FRS levels
and use the built in, not removable antenna on the FRS channels. it is short
(really short) range.
Option 2 is to go and get your GMRS license. Then get a handheld
that puts out the FULL 5 watts AND has a removeable antenna. Read over the
rules as to power output (EIRP) allowed. Set up an external antenna with a
max height of I think 20 feet above an existing structure(again check rules
on this figure) Take into account antenna gain, cable loss and ACTUAL
transmitter output to figure ERP. This is considered to be a Small Base
Station according to the rules AND is fully legal to interopreate with FRS
radios. The higher external antenna is almost guaranteed to quintuple your
coverage area from base to FRS handheld.
2 base stations with directional antennas(again watching erp) would be
really reliable.

Chris
KC2BZH