They should move him close to the parents. Ideally, next to one of them, the other parent seated with the other two kids.
They would charge a fee if no parent was traveling with a 7 year old, because by their own rules she needs care that can't be expected from a stranger sitting next to him by chance. They wouldn't let her travel unaccompanied by an adult, so they charge the fee to accompany the minor child.
Finally, my main problem with seperate seating like this is in an emergency, the parent is likely to impede other passengers as he wants to get his kid to safety - you know, be a parent. FAs will nag passengers to no end if p their baggage doesn't fit under the seat because it might impede other passengers in case of an emergency. This is far more serious.