If he was smacking a lot of children, some parents must have known of this.
Either they see marks on bottoms at bath or bedtime with young children.
A child telling their mother they were spanked or most likely a child telling their mother that so and so was spanked and that mother passing it on.
If nobody said anything about this, then one has to assume that it was regarded as reasonable.
When these cases come to court today they have to be based on what the law said AT THE TIME, not as it stands now.