Many thanks to all for your help. From what I can see of the seat map there are hardly any open seats so I'm guessing the flight is very full. Unfortunately the earliest we can get to the airport is 3 hours before departure - so it looks like we're stuck with what we've got.

The older child may be big enough to climb over a seat to get out to the aisle - but the younger one certainly won't be able to. Swiss Tony's solution of putting the children in 18EF and having them climb over one of us in 19D seems like the best bet.

The suggestion about next time putting one child on the Amex 241 booking, and getting one adult WT+ is a good one - although, if the loads turn out to be very high, then the advantages of OLCI might be diminished. I do wish though that, at the time of booking, the agent had pointed out the bear trap of not being able to use OLCI for the children.