I've done that ride a few times and the idea of taking my kids would honestly kinda scare me. There are many many inexperienced riders that do it and lots of mishaps (crashes). Although its not hilly per se I'm sure the hills would be very challenging to kids especially without gears. One more thing that I tell everyone that does the 5 borough though: If you do go, don't start with he pack at the ferry. Its a huge crowd and you wind up walking a long way if you're in the back. Instead plan to start up where the ride comes into central park. Just wait for the police to pass (stay out of their way) and then go behind them. You will have plenty of room from the crowd and can hit all the rest stops while they still have food :-)