If the carrier offered a facility whereby you could all sit together and you declined this, then that becomes your choice and clearly any such "guarantee" would not apply. So to that extent they were right.
I am not aware of any regulation that requires any minor to be in the care of a named adult. Obviously the primary care and duty of care is enshrined in you the parent, and perhaps with that in mind the first sentence of this reply would be of some relevance ? Airlines do carry minors (accompanied and unaccompanied) in accordance with their own conditions of carriage, however that does not normally stipulate that the minor must necessarily be seated with any named adult, other than in the case of an infant or a severely disabled child or where special arrangments are a condition of carriage, or have been arranged in advance.
From a regulatory or contractual point of view I think you have little or no recourse. Having said that, I strongly agree with the basis of your argument. I believe it is completely inappropriate for parents to be seated seperately from their young children. In the event of an emergency it is not incumbent on any other adult to have to take responsibility for somebody elses child and particularly when the parent is travelling on the same flight. In the same vein, an evacuation might well be compromised by a parent or parents having the natural imperative to search out their own child and impeding the flow and hence the likelyhood of a successful outcome. Similarly in a decompression. Finally of course no child should be seated next to a stranger simply to satisfy an airlines logistics. The consequences of any damage or injury caused to the child by any found contributory negligence on the airlines part, may lay them open to liability. Unfortunetaly it is a level of risk the carriers seem willing to take.
I think many airlines fail to pay sufficient heed to the requirements that wherever possible families should always be seated together or in very close proximity. Having said that I am sure that when deciding seat allocation in advance, the family situation is a consideration. On flights I operate, I do consider this of importance when it is brought to my attention, and cabin crew will always do their best to achieve a resolution or at least the best possible compromise. Of course they and I would prefer that this was resolved at check-in, but this isn't always possible.