Don't -- couchsurf with opposite gender hosts. A few girls I know regularly couchsurfed...it stopped when they woke up to a dude smelling their hair & used clothing while doing other "inappropriate" stuff (by himself thankfully). Interpretations of "payment" differ from country to country.
I've also heard horror stories of hosts who have multiple other roommates -- none of whom agreed to allow random people to sleep over. Nothing like being evicted at midnight in a foreign country where you don't speak the language, because the other tenants got back from a night out and want their bedroom back.
Hostels aren't too expensive, even in Europe, and if your kids are traveling together or with other friends they can probably get a private room (4 walls & a door that locks all the crazies out) for 20-50 Euro together depending on where they are. Definitely worth the piece of mind in my book.
