Adults, by choosing to live in a society that punishes non-cooperators, implicitly accept a social contract that allows them to be punished similarly. While they would prefer not to be punished, most societies don't offer asymmetrical terms, or impose difficult requirements such as elections, on people who want those asymmetrical terms.

Children, on the other hand, have not yet had the opportunity to choose the society that gives them the best social contract terms, and wouldn't have sufficient intelligence to do so anyways. So instead, we model them as though they would accept any social contract that's at least as good as some threshold (goodness determined retrospectively by adults imagining what they would have preferred). Thus, adults are forced by society to give implied consent to being punished if they are non-cooperative, but children don't give consent to be eaten.