Where do I enter my child's interest income for savings when did not receive a 1099-INT?

It's fairly straightforward.  Please see the TurboTax Help article What if I didn't get a 1099-INT from my bank? for guidance.

Also, you're not really specific as to whether you're reporting your child's interest income on his or her own return, or yours.  It's often easier and less expensive to include your child's income on your own return, rather than have them file their own return. 

Please see the TurboTax Help article Should I Include a Dependent's Income on My Tax Return? for guidance.