get more seats. if it is a long haul flight having 2DC's on laps is not ever going to be fun for you. definitely try and pick seats if poss. ring the airline and tell them that you have 6 kids and ask for this to be taken into account. really would get at least one if not two seats as being on your lap and having to deal with other LO's bears no comparison to being 2 adults with 1DC.
Do they have bassinets? The infants may be able to go into those. Some airlines also have a strange policy (ba) about how many infants can sit on a row. I think ts about the oxygen masks. (Dh and I weren't able to sit on the same row when we flew with our 2 infants). That would mess up your setting arrangement! Best check with airline about that. How old are the others?
The extra seats are a good idea and also try and get extra leg room so there's space for the children to sit on the floor and play if they wanted, it will also give you space to put things to hand for them.
Do you have an iPod or iPhone for the older children to use? You could download loads of apps and things tats sure to keep them entertained.