It's almost always better to file a separate tax return for the child. You might end up paying more tax if you include the child's interest income on the parents' tax return.
Form 1040-ES is not a tax return. All tax returns are filed on Form 1040. If you use TurboTax to prepare the child's tax return, TurboTax will fill out the proper forms. If you are using TurboTax Online, you should be able to use TurboTax Free Edition for the child's return.
