i guess if there are only six weeks and more than 8 people, its simple maths to work out demand could outstrip supply. If there is anyone with 2 weeks, they should give up a week in my opinion. it's pants isn't it? i dont have any children and am constantly getting the rough end of the stick when it comes to holidays etc at the hands of my childed co workers. I would imagine if i had children i would feel very differently though!
however i dont really understand your comment about not knowing when he can have time off as christmas is only 2 days, 1 week at the most, and there are 51 other weeks of the year to chose from.
Anyway, i think the solution is to look at who had what last year and those that didnt have summer should get first chioce this year.