I think it reasonable that I should be responsible, but I cannot perform that task if a third party's policies, in this case an airline, mean that my children are allocated seats in such a location I am not able to look after them.
Just book them in as unaccompanied minors on the same flight next time ... then you can put the responsibility boot firmly on the side of the airline...
Paid seat allocation has been around for years, not exactly like its a new thing that has just cropped up in the last few months.