S.S.
S., 4 is pretty young for sleepovers that involve a lot of hooha. I would suggest an activity but pretty low key. and some calming stuff when it comes closer to bedtime. the nails and hair thing is cute idea. and then maybe movie/snack and a story before bed. don't let it get too late or your going to have overtired preschoolers which will then turn into "I want my mommy" as far as goodie bags go is it a birthday party sleepover? if not why the extra's. just make it a fun night and leave it at that. I would suggest you have them all bring sleeping bags and their own pillows so they are comfortable at night time.
S.
oh yeah also from having experience with sleepovers as I have a daughter. make a set time the next day for pick up. or you may end up with extra kids all day long. you may not mind if there all family but it is always better to have a set time. like 6pm till say 9 or 10 next day or whatever but better to have a time than not.