You might end up on a 320 it depends on how busy the flight is. I'd go for a row near the back for the access to the toilets and you can lift up the armrests incase your kids want to lie across e.g. your lap! When you get on ask for a blanket and pillow as there are very few loaded on board and go very quickly. Wait until your kids feel a bit chilly and you will probably find they've all gone. Possibly the only problem with flying GB with kids i.e. lack of blankets.