Hakeemul Ummah Hazrat Moulana Ashraf Ali Thaanwi Sahib رحمه الله has advised that if a child requests something, fulfil the request promptly. If the request is refused, then regardless of the tantrums of the child, never then fulfil the request. If the request is fulfilled after the child’s obstinate demand, he/she will develop the habit of making obstinate demands.
(Malfoozaat of Hazrat Moulana Ashraf Ali Thaanwi Sahib رحمه الله, Part 7)
NOTE
The request refers to something which is permissible and valid; not something which is Haraam or harmful for the child.