I have some little Cobra MicroTalk FRS radios. They don't use the GMRS channels so no license needed, they are tiny, using 3 AA NiMh rechargeable batteries each, and charge via a micro USB port. These can be charged using any typical cell phone charger, and they came with a USB splitter cable to allow both radios to charge from a single USB port at the same time.
They are tiny radios, hardly larger than a 90's era pager , and narrower than a deck of playing cards, minus the antenna stub. Very simple operation that a kid can easily learn, and they also receive NOAA weather channels. I think they were $29 a pair at walmart.
The things that typically are limiting factors for FRS (very short range and low power) actually make these a pretty good choice for kids radios, provided your kids are obeying the rules on-air. Even though their range in a typical urban or suburban environment will be severely limited, they will work sooooooo much better than the 27 MHz or 49 MHz toy walkie-talkies you might remember as a kid. They will usually work great within the same city block or suburban neighborhood.