where do I enter amount contributed to 529 plan for my grandkids?

You do not get a deduction for a contribution to a 529 plan on your federal tax return.  The tax benefit of a 529 at the federal level is that the earnings on contributions are not taxed (as long as used for qualified expenses).

But, some states allow as deduction for 529 plan contributions.  If your state allows a deduction for a 529 plan contribution, you'll be prompted to enter it when you prepare your TurboTax state tax return.