I know that many airlines have a fee that must be paid in order to choose a certain seat, and if you don't pay, then you have one randomly assigned to you.

How does this situation work out with an adult traveling with say, a 5 year old, if the fee is not paid?

Would the airline tend to make an exception and make certain to seat those passengers together, or would it insist on the payment of the fee to reserve the seats?

All help appreciated.