I don’t think it’s difficult to accidently kill someone - especially a child. They could drown unattended in the bath (one of my theories), they could hang themselves while unattended, eg in the cords of window blinds, they could bang their head or suffer an injury from a reprimanding blow from a caregiver, they could eat something they shouldn’t while unattended, they could choke, they could drink a noxious substance, etc.
Also, I do believe that some people would cover up an accident. Easier to say a child has wondered off than you were responsible for their death.