Spanking

It is not, however, permissible to strike a child, or vilify him, for the child’s character will be totally perverted if he be subjected to blows or verbal abuse.

‘Abdu’l-Bahá, The Compilation of Compilations vol. I, p. 289-290

As to your question about the use of physical punishment in child training, although there is a Tablet of the Master which considers beating as not permissible, this does not necessarily include every form of corporal punishment. In order to have a full grasp of the Master's attitude towards punishment, one has to study all His Tablets in this respect. For the time being no hard and fast rule can be laid down, and parents must use their own wise discretion in these matters until the time is ripe for the principles of Bahá’í education of children to be more clearly elucidated and applied.

Shoghi Effendi, Lights of Guidance, p. 151

While the physical discipline of children is an acceptable part of their education and training, such actions are to be carried out ‘gently and patiently’ and with “loving care”, far removed from the anger and violence with which children are beaten and abused in some parts of the world. To treat children in such an abhorrent manner is a denial of their human rights, and a betrayal of the trust which the weak should have in the strong in a Bahá’í community.

Universal House of Justice to an individual believer, 4 August, 1996