I second the notion that bulkheads are great at that age because toddlers and preschoolers are always kicking the seats or messing with the trays and it's trying to have to be vigilant the whole time. Plus the bulkhead gives the younger one some time off lap to stand up and stretch a bit, which can be nice.

But I, too, would trade off all those advantages for the movable armrest, especially if I had to pay extra for the bulkhead (which for some reason I thought the original poster mentioned...).