In all likelihood the crew wont care so long as the 2 children have an adult looking after them/boarding with them. e.g. If the 3 x economy seats you have are all together and include your eldest (over 16) then id have your 3 children take their seats in the back whilst you take your seats in the front. Like others have said I wouldnt mess with the ticket, nor would I order special meals (except perhaps for the child that IS in their original seat) - as it only brings attention to the situation, just pick some kid friendly food up at the airport for the displaced kid.