There were only 3 men.
One was the grandfather aka the father #1
One was the father to the child and simultaneously the son of the child's grandfather (father #2 and son #1)
One was the child (child #2)
If there were two dad's and two sons, and they each bought an ice cream cone worth 50, but only paid 1.50, that means there was a grandpa, who is a dad,
a dad, who is a dad of the child and a son of the grandpa,
and a son, who is a son of the dad.
This all adds up to two dads and two sons.
(the dad is both a dad and a son, if you don't understand ;))
