The child attachment belt works well in moderate turblulance or minor "quick stops" but in a serious deceleration (think crash test dummy video) the adult will double-over crushing the baby while the baby causes serious internal injuries to the adult.

The safest place for a baby is to be strapped into an aircraft-approved child seat in his own seat. Anything else is a compromise and no single compromise solution provides a clear advantage in all scenarios which is why different regulator agencies have come up with different rules.