What is the ruling on burning a small child with a candle, a lighter, or an iron to discipline him and wean him off diapers?

Typically, a mother shows mercy to her children and is keen on what benefits them, avoiding what harms them. This is supported by reason, scripture, and reality, for Allah is more merciful to His servants than a mother is to her child. It is unlikely for a mother to harm her child in the manner mentioned in the question; rather, it would merely be a form of intimidation. However, she should not do that, even if only for intimidation, because it can lead to negative psychological effects on the child. A wise mother will not be devoid of methods and means by which she can influence her child and make him accept what she wants to do towards him, including engaging him with some permissible forms of amusement.