These phrases are quite different, and I wouldn't interchange them. I'd say "in their best interests" is stronger language than "for (or 'to') their benefit." For their benefit means it would be nice, but it's optional. "In their best interest" suggests bad consequences if they take another path. "Benefit" comes from Latin "bene" meaning well, in a good way. "Best" is English, and is the superlative form of "well" or "good."