Longman Dictionary doesn´t say that slumber party is only for girls, from what I read in the entry "slumber party":  Am. English => "a children´s party when a group of children sleep at a children´s house."

      I have seen at Google Image, that today slumber parties can be thrown by and to not-so-children; teens have their slumber parties as well [though, not so commom. It seems.]

     By the way, slumber is the literary word to sleep.

     In Brazil, recently, there´s a tendency to schools (private ones) hold pajamas parties.   Of course, their parents are the ones that foot the bill.

     Pajamas parties, Brazilian way; is one where the little ones get (even loud) music,  snacks, and other activities (fun, fun, fun), they go to sleep and the next day (mostly Saturdays) they get their breakfast and go home.   Teachers and party organizers are the ones responsible  and the school is the guest.    That´s it, if I am not missing something.

PP.

    See also

http://en.wikipedia.org/wiki/Sleepover
A sleepover, also known as a pajama party or a slumber party is a party most commonly held by children or teenagers, where a guest or guests are invited to stay overnight at the home of a friend.