I'm leaning toward telling her that she can have a similar "do" to the Australia Day event: 20 kids. From midday to 6pm. BBQ, sausage sizzle, salad. And a strict door list.
Now that is a very smart move I think. Sausage sizzle for them is a great, and 6 hours is plenty of time.
Back when I was 16, a friend had a party for her 16th and invited 20 friends. In excess of 300 kids turned up and totally trashed the place. Furniture was stolen, cars and a motorbike were stolen (one was found down the road burnt out). And this was way before the days of Facebook to invite people. The kids just heard it from word of mouth. So I think you have to be really careful when inviting kids to a party.