I know it is of no help what soever but that is just how the system works. A computer is not capable of making the difference between people traveling with children and those without. And the programming would lead to complexities. As far as I am aware there is no airline which is able to provide such a service via their computer systems. Thats just the way it is. It should be able, but it is not, no one has designed a system which can! I am sure you would find the same problem with most airlines.
Best bet if you really want seats together is to either pay a higher fare or turn up bright and early at the check-in desk to secure yourself seats together.