Take your child grocery shopping with you. Let them make some controlled choices, like selecting between Gala or Fuji apples, or different pasta shapes. If your child asks to get things that are not on your grocery list, talk to them about wants versus needs. You NEED nutritious foods like apples to be healthy and strong. You WANT treats like cookies.
God supplies all our needs.
Understand the difference between a want and need, and the need for making good choices.
And my God will meet all your needs according to the riches of his glory in Christ Jesus.
Philippians 4:19 (NIV)