Wow, this could be a complete nightmare!

Do you have anyone you could send the boys too for the night? Like grandparents/aunties/uncles/good friends?

What about if you suggested she turned it into a beach party? Make it during the day and they have heaps to do at the beach, sunbaking, swimming, take some balls down to the beach etc. For food you could have a couple of eskies of soft drink and make sure the beach you pick has a BBQ area so you can cook up some sausages, make your own patties and buy a huge stack of bread rolls. If the beach is a fair way away from you all the better, it will minimise the number of kids who will come, it might also seem cooler to them?