It makes a certain amount of theoretical sense, and, yes, you’re off-base on this. First, parents (or other guardians) have a joint full responsibility for the child. If they agree on a 50-50 split, and one of them (for any reason) does only 25 percent of the work at a given time, the other one can’t just stand back and say, “I’m doing my 50 percent.” The _work_ may be split 50-50, but the _responsibility_ is split 100-100. I just don’t see it making sense, socially, legally, or in any other real-world way, for an employer to be in a position to say, “Yes, your child has a high fever, but we think your spouse should take a sick day instead, so the kid’s just going to have to sit home alone and uncared for.”
That brings me to the second part, which is the extraordinarily offensive notion that employers should be able to micromanage their employees’ lives for maximum business convenience. Would you have to keep a log of child care activities to prove that you and your spouse shared the work equally? Bring it up to some kind of corporate hearing board in case of disputes? Sick leave and other forms of time off are specifically designated for illness of self or other family members. For employers to pass judgment on which family members should be allowed to take time off for bona fide illness at the very best invites a counterproductive bureaucratic mess. Thinking in such narrow, ompartmentalized terms is, btw, traditionally considered a male, “rational” activity. Determine what the childcare jobs portion is. Even if the employee IS divorced, suppose part of the responsibility of the non-custodial spouse involves helping out the custodial one if the kid is sick, so that one person doesn’t have to miss work all the time.