There are a few solutions to this. You could completely separate health insurance from employment. Or you could require benefits for part-time employees as well.
I never got why health insurance is tied to employment. Then when you switch jobs, you start a new plan and then there could be a gap where you could be uninsured. I think health insurance should be separated from employment. I think we're the only country that does it this way.
You can also do things like require the same sort of benefits for all employees. Pro-rated vacation and sick time, if offered, as well as retirement, discounts, and other such things.