Teens in Italy mostly look like teens in the States (maybe a bit more formal, but not always). Italian pop culture borrows heavily from American trends. Still, shorts are not appropriate in church. No one will stop you but it’s a matter of propriety and respect. Churches once intervened but the times they are a changin’. Have teens bring at least a pair of long pants for any “formal” occasion that might come up. Also, bear in mind that while Italian teens (and young women) may seem ultra-casual, there are old-fashioned cultural barriers behind the cool look. Italians will never pressure you about what you wear but they’re always pleasantly surprised by well-dressed Americans. It’s your call.