In my experience, an 11yo boy is likely to be happy to sit alone in a seat near, but no necessarily immediately adjacent to the rest of the family.

Alternately, put the kids together with the other adult and allocate yourself a separate seat at the other end of the aircraft. just remind the FA doing the CityFlyer beer delivery that you have paid the same fare for the two kids and since they are under age they cannot partake in the beer service, so would she be so kind as to leave their cans of beer with you and you will ensure they are kept away from the minors.