As soon as you start entering data in the new W-4 tab, Sage will ignore/remove the dependents from the FIT row on the calculations tab (if it was stored there prior to the new form)

You still need the marital status and dependents (qty) for the state row for SIT

so if you check the box but don't enter any deductions or dependents what happens to the dependent numbers?

I was referring to the calculations tab where the rest of the taxes are also listed.  There is no box to check there.  If you are referring to the new W-4 tab, then you do need to complete the form.  The dependent field in Step 3 is not a quantity anymore it is a dollar figure.  If you or your employees need help completing the form, there is a free IRS calculator that will produce the correct form after you answer a series of questions.  

https://www.irs.gov/individuals/tax-withholding-estimator