Walkie talkies may or may not work. If you don't have them already I wouldn't buy them though.

Best non-technical suggestion I have is post it notes on the door of one cabin (for a large group pick a single cabin door as the post it note door). "Mom, we're going to meet in the buffet at 1pm" "Kids, we'll be by the adult pool until 4pm and then we are going to the martini tasting in the SkyBar at 4:30"

My cruise roommate and I use this method all the time. Plus the post its come in handy if we need to leave a note for the stateroom attendant "can you please bring me some ice for this champagne?" etc.

What ship/cruiseline are you sailing? We may be able to give you more specific suggestions if we know that information.