pester power 
noun [mass noun] informal
The ability of children to pressurize their parents into buying them products, especially items advertised in the media.According to Anne Smith, grandparents often prefer to buy traditional dolls and are less influenced by advertising and pester power than parents....
- A spokeswoman said: ‘£30 for a kid's shirt is a lot of money but it is all about pester power and for parents to counter that and say no to their child if they think that it is too much.
- Parents try to resist pester power as they walk around supermarkets full of sugary cereal with toys inside the box or tinned spaghetti displaying pictures of a popular band.