I understand that if you are a single parent or your DP is otherwise engaged/working, and if you have no family nearby, you may need to take all your children to a party which only one of them has been invited to. I have had to do this a number of times, although I make an effort to make alternative arrangements if possible, especially if the party is being held in someone's house/small venue.
But why do whole families (Mum, Dad, and all children) go to parties together, when obviously only one sibling has been invited? Presumably (BF babies apart) one parent could stay at home with the uninvited siblings, whilst other parent takes the invited DC?? 
I just don't get it. Its totally inappropriate and selfish for parties at peoples (small) houses, and also really odd when older siblings come along to parties completely unsuitable for their age (I'm thinking of soft-play parties where both parents have come and dragged along pre-teen DCs!!). I can see the point of taking age-appropriate siblings to soft play parties (paid for spearately), with DP to help out, but the rest of the time its bonkers IMO.