I can set pretax money aside through my employer for child care expenses, subject to the terms of IRS Pub 503. Does a non-working spouse disqualify preschool expenses?

Both spouses generally need to be working for the employee to have daycare expenses that are eligible for dependent care FSA reimbursement.  Exceptions apply if the spouse is looking for work, a full-time student, or incapable of self-care.

@jared123 a non-working spouse does disqualify you.  the whole idea of the tax credit is so working parents can get help with expenses to watch the child while both are at work.  There is no need for that support if one parent is 'stay at home'  is the way Congress thinks about it.

is your spouse seeking work (and the spouse must find work by year's end), or in college, etc? 

Thank you both

You cannot get the child and dependent care credit on your federal tax return unless both spouses show income earned from working.   If your spouse does not work, then you might get the credit if the spouse is either disabled or a full-time student.   If the spouse is not disabled, not a student, and is just a stay-at-home parent, sorry, no childcare credit.