You should have every model you work with sign a model release, no matter how much or little of the person is visible. A signed model release protects you and Stocksy from any potential problems. Additionally, some clients won't buy unreleased images, even if the person is unrecognizable or only hands and feet are visible, so if you can get a release, get one.
A release is legally required if a model is recognizable in any way. Even if you can't see someone's face, that person might be recognizable due to other contextual clues like tattoos, unique clothing, location, activity, or other visible people.
At Stocksy, we occasionally accept unreleased images after our Legal team reviews. We don't accept any unreleased children.