why the necessity for sleep overs ?
for what ?
better to keep informed on what's going on over at the schoolhouse in keeping them innocent as the crap they introduce/already are cramming into them are never "age appropriate""
and not that long ago would be classified as PORN.
simply tell them that their underwear area is off limits and how to deal w/it,
IF in a grammar sch/kindergarten/pre-k setting,find out what their rules are for the bathrooms,any supervision,who supervises, do they have a "buddy system " things like that and keep your family photos and theirs OFF social mediyuuh.