16 and 20 are practically grown ups. I think you shouldn't worry too much about them being entertained. At 17 I was travelling alone and had no trouble finding things to do. Just send them to Margaritaville with $50 each and let them hit on the tourist girls that pull up in the catamarans.
at the risk of my post not being helpful, I use my Jamaican money clip (binder clip) to hold my bills in my pocket. To me it feels less conspicuous than a dangley thing around my neck that says I have money in here. Sorry to the merchants if I have wet bills, their choice to take it or leave it.