What clothes should a teen pack as far as shorts, leggings, dresses, skirts and tops. Can they wear short shorts and tank tops?
They can wear whatever they want, except in churches in Italy, Spain, and possibly other southern countries. In many churches there is a dress code that says knees and shoulders must be covered. Leggings and a short sleeve top would be fine, but shorts and a tank top wouldn't. A dress to the knee would be fine, if it's sleeveless, a scarf or cardigan put over it for the visit would be fine.
When I was a teen I went on a trip to Greece (this was 15+ years ago, so fashion has changed!), and all the Greek kids made fun of us for wearing denim shorts. Shorts just were not done. I think this has changed, especially for young women/teens. However, if you are going to Italy, short shorts may get them more attention than you or they may be comfortable with. Especially if they are older teens or just look more like 19 than 14. Unfortunate, but true. Not a safety risk, just an uncomfortable annoyance. Are we talking 1 inch inseam daisy dukes, or a more moderate 3 inches? I wouldn't advise the former. They will see more local teens in sundresses than in shorts, but other that in churches they will be fine in whatever. Teen girls are notoriously picky about fashion and this may not be a battle to fight.
I cannot emphasize how important it is to pack light enough so each person handles their own luggage.
My wife's no teen, but she travels with jeans and other clothes that are either black or tan. She's not into sun dresses, leggings or anything approaching style. Traveling light may include her carrying only one skirt and packing 1 extra pair of shoes.
And everyone's right about Italy and other places with a lot of churches. The doors are guarded, and they'll ask you to leave with bare legs and shoulders.
They can carry what they feel comfortable in as long as it is decent. A girl in short shorts and a revealing tank top might be seen has having loose morals by many European men who might think she is ripe for sex with them and harass or pursue her. Sure, tourists do that but in many places (Spain, Portugal, Italy, Greece, Turkey, France, etc) this is an open invitation. No, it isn't fair but life isn't fair. They should dress to fit in with Europeans, not in a way that screams "American tourist" which can seem like an invitation to thieves as well.You can't go in the churches and cathedrals dressed that way nor in the Vatican. It would also be inappropriate at nice restaurants, museums, the symphony, or opera.