AMs vote on introducing a ban in the Senedd next week.
The issue has caused tensions within the Labour group, with the Welsh government ruling out changing the law before next year's assembly elections.
One Labour AM, Christine Chapman, spoke last year of feeling "cheated" that ministers had not addressed the issue.
In a joint letter published in this morning's Western Mail, the Most Reverend Barry Morgan, Professor Sally Holland and Phil Bale, leader of Cardiff Council, argue that a ban would simply give children the same protection in law as adults.
They write: "We call on the First Minister and Welsh Government to allow Labour AMs a free (conscience) vote on an amendment to the Violence against Women, Domestic Abuse and Sexual Violence (Wales) Bill concerning the physical punishment of children, and we call upon all AMs to support this amendment.
"The amendment would remove the defence of 'reasonable punishment' in relation to assaults on children in Wales and finally give children the legal protection from physical harm that they deserve."
Labour's current position is that there will not be a free vote for its AMs on the amendment, tabled by Cardiff North AM Julie Morgan.
The Archbishop of Wales and the new Children's Commissioner have led calls for AMs to be given a free vote on a ban on smacking children.