Hi GlennM, and welcome to cruise-chat.com!
It's been a few years for us, but our son enjoyed the kids program on HAL. It's not as extensive as those on Royal Caribbean, yet he is a low-key kid who was happy just to hang out with other young ones his age. (This was in 2004, and he was 10 at the time.) Still, he did quite a bit, from sports to making a tie-dyed T-shirt--and came home with a lot of stuff.
If you are going the week of April 20, there will be many kids on board as it is a school vacation week for many in the Northeast U.S.