I do not have teenage siblings or kids. My kids are in the kindergarten age or younger. However I have been thinking about rewarding with a quarter/nickel for everytime they pray (even though they do not completely know how yet). Probably a nickel if they do the actions and a quarter once they learn. However it will not be only for the money. I will also explain to them this reward is from your parents however Allah SWT also rewards in much more than nickels and quarters. This is so they do not learn to do things only for money but enough to provide incentive and get them in the habit of saving. At this young age they need to see something tangible.