Originally Posted by SilverChris

Have a look at this, as it directly addresses your split-cabin plan. You can't get your two children ticketed in Y if neither of them is at least 16. I'm not sure why you'd want to purchase three Y tickets, because that seems to be overcomplicating matters.
https://www.qatarairways.com/tradepo...assengers.html
From a ticketing standpoint, your intention of booking the adults in J and kids in Y wouldn't work.

If I were you, I'd just keep your tickets as-is and sort it out onboard. Ordering a kids meal in J would complicate the seat switch. They can't serve a J special meal in Y. You can order the kids meal for both passengers in Y - I'm fairly certain there's nothing stopping an "adult" from ordering a kids meal.

Once onboard, just casually tell the crew your kids are seated in Y, and emphasize that they won't be coming up to J to visit, just the other way around. I'd imagine they'll be accommodating enough for that. If they insist on enforcing the 16 year old rule, then I'm afraid there's nothing you can do but comply.