My wife has a Giant Cypress in a 15 1/2 inch frame with 700 wheels. Frankly, even that size bike may well be too big for your kids. I would go with 26 wheels. My wife's Novara has 26 inch wheels, as does my rigid Trek MTB which I have converted to a tourer. I see no advantage to a 700cm wheel. 26 inch tires and tubes are readily available.