Thanks to all. I think I'll get 4 economy seats. Even if there is no availability, I'll buy one or two.
BA is an option, but the connection time for me is 1 hour. I was able to do it only running through T5 with all the lines, taking off shoes and such. With 2 babies, no way. Or I have to make a stopover in London for a day. Besides, the travel time is longer and the flights cost more Avios. I know what to expect from BA, but have never flown Iberia. Chose it due to good redemption availability and shorter flying time.