Sorry, Cote but if you buy two plane tickets, you are buying two seats somewhere on that plane - not two specific seats. You have only bought two specific seats if you have paid extra to pre-book specific seats, or unless the ticket price specifically includes pre-bookable seating and you have taken advantage of that, and chosen specific seats.
I have to say that I think airlines could and should do more. If I book online, I have to put in the ages of any children travelling with me - and it is not beyond the wit of mortal man to design software that will allocate each child on that booking a seat next to an adult - and to ensure that 'next to' means a seat in the same row, not separated by the aisle.
And I agree with the people who believe that airlines aren't doing this because they think people will pay extra to ensure they do sit with their children, and that this is a nasty policy on their behalf.
But I also know that, whether I like it or not, if I choose to fly, I have a choice - I can either pay extra to pick my seat, or I can pay for a pricier ticket that allows me to go into online check in and select my seat, or I can take a chance that I won't get a seat that fits my particular requirements or likes and dislikes - so I choose to play within the rules, and pay the extra. That is my choice.
But if someone else chooses not to pay the extra, and would rather take the risk of sitting apart from their child, why is it then my responsibility to rescue them from the negative consequences of their choice?