Money cake
My Mom always baked frosted sheet cakes for our childhood birthday parties. In addition to party favors, she would bake quarters (a fair amount of money for a child in those days) wrapped in waxed paper and strategically placed in the pan. Then she carefully cut the cake so that each child had a coin at the bottom of his/her cake. I always liked that. I remember watching her leaning over the cake with the cutting knife in her hand, mentally measuring pieces before she sliced, to be sure the pieces were all uniform size and proper dimensions to guarantee each child had a coin.
I know in some families only the birthday child got the money. Other mothers would bake different denominations into the batter - quarters, dimes and nickels. Some mothers might bake money cakes for different occasions - like Christmas, for example. But my Mom only did it for birthdays.