This question has multiple solutions. Unless the question has another restriction ( like the youngest child has blue eyes which means the age of the daughters can't be the same if they are the lowest )it is unavoidable.
2,6,3 = 36 => 11 ( seat number )
2,18,1 = 36 => 21 ( seat number )
12,3,1 = 36 => 16 ( seat number )
6,6,1 = 36 => 13 ( seat number )
9,4,1 = 36 => 21 ( seat number )