I know this is an old thread but I thought I would chip in (better late than never, right?)
Last year my mum and I threw an adults-only party. Since pretty much everyone we invited were adults with no children or grown-up children, we didn't expect there to be an issue. The only people to consider were my brother & his wife, who relish the chance to have a night away from their toddlers, so that was fine, and my stepsister & her husband. My mum was very clear when she invited them (2 months in advance) that it was an adult's only party with drinking and adult games so it would not be appropriate to bring their 7 year old daughter and to make other arrangements for her. They said it would be fine, that she would stay at her friend's house that night.
So, day before the party, we get a phonecall saying that they are bringing their daughter because the sleepover has fallen through and they have nowhere else to take her. My mum and stepdad tried to explain that this was an adult-only party and there would be no children for her to play with and that she was too young to participate in the drinking games etc. Well, my stepsister is a handful at the best of times - she constantly falls out with her dad over the slightest thing, and she is very controlling. At the hint that perhaps if they couldn't find a sitter, they should come another time, she went mental at him and said that she had bought and prepared over £50 worth of party food and bought all the paper plates, napkins etc.
We were shocked to say the least - we had not asked her to do any of this?? Anyway, needless to say they caved in to prevent a family feud and they ended up bringing their daughter. To make matters even worse, my other brother decided to invite his work friend, who didn't RSVP, so not only did we not account for him but we certainly did not account for his wife and 2 very young children!!! They all turned up in costume and my mum was livid but couldn't turn them away. So we ended up with 3 young children running around bored out of their minds whilst their parents allowed them to get into everything. My stepsister's daughter is quite well-behaved but the other guy's kids were a nightmare. My mum had to tell him off several times for breaking things/making a mess and he was very rude - he kept buzzing round me and my friends and being quite spiteful at times and generally quite obnoxious.
Urgh. Needless to say, we won't be inviting my brother's workfriend to any future parties *rolls eyes*.