He claims he learned it when he was their age and hoped it would get them to change their mind and not vote him out. It was unsuccessful in this. Later in the episode however, he is able to force Peg and the kids to sing it in order to convince him to bat for them again.
D is for D many pies I make you
A is for the apple in my eye
D is for D dish you ate the pie in
D is for D apple of my eye
Y...because I love you...
Put them all together...
They spell... "Daddy"