Learning children to wait for the good thing is an important life skill. The typical game is giving a child (between the age of 3 to 5) 3 sweets, and tell her if she does not eat the 3 sweets by the time you come back for the shop,0.5 hour or so, she will be given another 3; if she ate the 3, then there will be no extra given.
Get them do the home work first, because it is something they have to do.
Just think, when they grow up if they have $500 in their pocket, would you want them to use it to pay their mortgage or go to a night club have a good time?