Assume that the child sits in seat 1
Count the number of ways that the child CANNOT sit next to either parent
They must sit in seats 3, 4 3,5 or 4,5
They can sit 2 ways in either seats 3,4 , 3,5 or 4,5 and for each of these, the other people (not counting the child) can be arranged in 3! ways = 3 (2)(3!) = 36 ways
Total arrangements = 5! = 120
So.....the ways the child CAN sit next to at lrast one of the parets = 120 - 36 = 84
{ The Guest and I agree !!! }