Yes, minors often need extra paperwork beyond their own passport and visa. The most common requirement is a notarized consent letter when a child travels with only one parent, with a relative, or alone; several countries actively check for this at the border. The letter typically includes the absent parent's consent, passport details, and the travel dates.
Each child normally needs their own visa too. Kids are rarely covered under a parent's visa these days, even where fees are reduced or waived for young children. Some countries also ask for a birth certificate to prove the parent-child relationship, especially if surnames differ between parent and child.
Airlines have their own unaccompanied-minor rules that are separate from immigration requirements, so check with the carrier as well. Since requirements differ widely by destination, confirm on the embassy or official immigration website of the country you are visiting before booking anything.