It depends on:
If your part-time employees meet certain conditions, then they could be entitled up to 12 weeks of unpaid leave after the birth of their child.
Employees are covered under FMLA if they:
Certain states have laws that add to the FMLA.
For example, in the state of Washington, a pregnant woman is entitled to the 12 weeks of FMLA, plus any pregnancy disability leave ordered by her healthcare provider. So if a woman in the state of Washington is ordered by her doctor to take 6 weeks off, she can extend her maternity leave by an additional 12 weeks of FMLA, for a total of 18 weeks off.
For additional state-specific resources, you can perform an internet search of law on maternity leave for part-time employees to see if your state has any laws that expand upon the FMLA.
The U.S. Department of Labor's Website Covering FMLA