It does not get any easier or simpler than a set of GMRS hand sets (walkie-talkies) from Walmart etc...you can use the FRS frequencies without any license required...plenty of range for the described situation...don't know why others are making it sound more complicated...
For starters, it is illegal to connect a FRS radio to an external antenna. Since users in this scenario would likely want to use such a radio indoors, the combination of low power FRS with no external antenna would severely restrict the radios ability to function as the OP wants. Using a FRS radio for boating or two fisherman keeping in touch along a stream is a much different usage.
I recently had friends buy two GMRS radios for emergency communications. Very similar desire to what the OP wants. I advised ham radios but they said that was too complicated. My friends spent several hundred $$.
I looked into their radios after the purchase. If I recall correctly, the GMRS radios are "channelized" so you select what channel to communicate on, not what frequency. This means you can only choose the channels your GMRS radio allows you to choose. I looked at the channel list for their radios, and found some low power channels, using very low wattage, and some "high power" channels. But there are only 8 high power channels. These people are in a metropolis urban environment. Do you have any idea what those channels are going to be like in a SHTF scenario, with hundreds of users all wanting to share only 8 channels?
It got worse. My friends believed they could have their "own channel" because the radio mfgr advertised you could set a "tone" for a discrete frequency. They misunderstood this to be setting up some sort of secret frequency only for themselves. Not correct. The "tone" only acts like a squelch, not opening the radio unless another radio transmits also using this same tone. But if the frequency is already in use you will be blocked by them, and you should not "step on" others if they are on the frequency anyway.
To the previous poster, this isn't "complicated" if you understand the facts and technology. I'm not singling you out ... only trying to share that there is a lot more involved than buying a pair of walkie talkies from Walmart. Someone with a ham license is going to better understand these tradeoffs typically better than average joe simply because they went through the training and probably better understand the issues at hand.
For those advocating UHF radios, one of the most popular models of mobile Ham radios is a "dual band" radio, permitting communication on 2 meter (fm) and 440 (UHF.)
To finish, the ability to have an external antenna, and choose frequencies instead of being restricted to channels, is a very big difference when it comes to radio use and performance.