Oh dear, things may not be as easy as you think. If you look at the Monarch website you can find seat plans of the A330 which I expect will be the plane you are taking, if the flight is shown as direct it will be, if it stops in the Middle East, it might be something smaller. The problem with the A330 is that the extra legroom seats are located next to the Emergency Exits and CAA Regulations do not allow children under 15 to occupy those seats. This means most, if not all, those seats will not be available to you.
Back problems and charter seats are not a good mix, whilst the tour operator might not have been willing to sell you a premium seat, the airline might be, you can go through the website and contact them to see if they will sell them to you direct. If not, the seats are around 30 inches in pitch, not the worst in the air, but not the best either.