You would get the excess SS tax withholding back on line 71 of Form 1040. (Although they're revamping the Form 1040 and moving a lot of stuff off the main form and onto schedules, so it'll probably be in a different location.)
Still, excess SS tax withholding is treated as a payment just like your regular withholding, so if you wanted to adjust your regular withholding down to account for it, you certainly could.
I personally like and have used
www.paycheckcity.com in the past, and it looks like it's still a good place to go. Basically all you need to do is figure out what 7.65% of your paycheck is and tweak the withholding until you get to that number.
If you want to count on your second child being born this year, that baby would generally qualify you for a second $2,000 child tax credit. To adjust your withholding for that, just divide $2,000 by the number of paychecks remaining this year, and tweak your withholding lower by that amount.
Finally, you'll need to remember to adjust your withholding higher again in approximately January 2019 by the 7.65% SS adjustment you're making, because that will be a one time thing and you don't want to be under-withheld for 2019.
HTH.