A stumble while you're holding their hands is the textbook mechanism for a pulled elbow.
Nursemaid's elbow — radial head subluxation — is one of the most common injuries in children under five. The mechanism doctors describe is specific: a sudden lengthwise pull on an arm that is straight and turned palm-down.
Now picture the moment your baby's knee buckles mid-step. Your instinct is to pull up. Their arm is straight, overhead, palm forward. That is the exact position, applied by the person least likely to be holding back.
A harness changes where you are holding them. There is no hand in your hand, so there is nothing to pull.