Differentnameforthis - I did not say your child did not have a right to sit in a seat you have paid for. Nor did I suggest they should travel in the hold (though I suspect some parents would pay extra for that option).
You said that, if someone who had paid for allocated seating did not want to move from that seat so a parent could sit with their child, then they would have have a nightmare of a flight, dealing with that child screaming, vomiting, demanding help and entertainment. That smacks of blackmail to me. It is saying, "I haven't paid for allocated seating, but I expect you to give up the seat you have paid extra to have and if you don't, you will have a horrible flight".
I do get that this is a problem that has been created by the airlines. They could make things better by automatically offering to refund the extra fee people pay to allocate seats in advance, if a member of the party has to move to allow a parent to sit with a child. That would probably make people more willing to help.
Or they could open only a certain percentage of the seats on a flight for pre-allocation, then they could allow parents with children (or others with specific needs) onto the flight first, to get first pick of the non-allocated seating, followed by those with allocated seats, and then the rest. That, I think, would solve things - people could pay to pre-allocate their seats if they chose, and parents would have reasonable confidence that they'd get seats together if they chose not to pay extra.
But if a parent makes the decision not to pay extra to allocate seats, then they are gambling on either being lucky enough to find seats together or on the fact that someone else will move for them - but that is their choice and their gamble, and if it doesn't pay off, they can't blame anyone else for this - under the current booking rules, of course.