'Pay attention' is a much commoner way to render this instruction in English
Pay attention would be translated as "preste atenção" or "prestem atenção" (plural). The person here is just saying "Attention boys!" which has a slightly different meaning. :)
Careful is closer in meaning, but there is also another word for that: "cuidado".
Yes "be careful, boys" or "pay attention boys" are better translations. Is it possible to add them?