QR are one of the increasing number of airlines that aren't interested in ensuring children are sat next to an adult. Etihad are the same amongst others I can think of. BA to their credit, one of the few good things I can find to say about BA, would ensure that the child (up to age of 12) was seated next to an adult.

You can try calling QR and pleading with them, depending on the agent you get they may at least accommodate to sit each child next to 1 of the adults in your bookings, but it really depends on the agent you get and the availability of seats not paid for/specifically selected by other customers.