We gifted our son and his wife $9,000.00 each for purchase of a home . Does this decrease my taxable income

Gifts given and received are not reported on a tax return.

You have to report the distribution from your 401(k) as ordinary income on your tax return.

To enter, edit or delete a form 1099-R -

Click on Federal Taxes (Personal using Home and Business)
Click on Wages and Income (Personal Income using Home and Business)
Click on I'll choose what I work on (if shown)
Scroll down to Retirement Plans and Social Security
On IRA, 401(k), Pension Plan Withdrawals (1099-R), click the start or update button

Or enter 1099-r in the Search box located in the upper right of the program screen. Click on Jump to 1099-R

You also have to pay a 10% early withdrawal additional tax penalty if you took the money out before age 59½. 

Gifts are nice, but never tax deductible.