We have used variously a Dawes Double Edge , Dawes Toucan, Orbit Libra, and two Thorn Voyagers, with our boys starting when they were 8 and 6.The best of all is a M/M Thorn Voyager which I still use with the oldest who is 13 and 5 ft 9. I am 6 ft 4 but use an adjustable stem to ensure my comfort whilst the boys seat has gradually been raised across the years. We were lucky that their legs have always been long enough to mnage. We have also had two spartas but they were a bit too heavy and scary on hills to be viable with the boys.
My boys still list cycling amongst their hobbies and still enjoy a trip on the tandem with Dad, it's an excellent introduction for them.