As far as I understand it the airlines cannot separate you from the children - you may have to sit, for example, one adult with one or two children with the other adult elsewhere, but they cannot leave the children on their own (although having four of our own, it is sometimes very tempting!!!) Even if they have to shuffle people around they will sort it out. Try not to worry - we have always been unable to check in online because (up until now) one of our kids has always been under 2 and you can't check an infant in early, and we have never had a major problem. Just try and concentrate on looking forward to your holiday. Have a great time!