Institute a paid time-off policy: Many employers offer 10 paid holidays, two or three weeks of vacation, and a set amount of sick days and personal days each year. In a paid-time-off system, all that time is lumped into a single bank of hours. Workers use that time however they need, without feeling the need to justify the time to the boss or to each other. That way, nobody resents the parents who need to stay home with sick kids for a couple of days, and childless employees who never get sick can reward themselves with vacation days.
If telecommuting is an option, make it optional to everyone: Sure, it may be easier to justify working from home with an explanation like, "I have to take my son to the dentist at lunchtime" than with an explanation like, "I have to take my parakeet to the psychic at lunchtime." But fair is fair. Employers should make it clear that their telecommuting policies apply to all.
Ditto, flex time: In a company with a strict 8-to-5 policy, a boss might be more lenient with an employee who arrives at 9:30 a.m. because he was up all night with a crying baby than one who arrives late because he was up all night crying alone. To maintain fairness, employers can institute flex time -- working a set number of hours, but not necessarily the same hours. Such a system benefits both the parent who arrives at 6:30 a.m. in order to be home when the kids get home, and the childless employee who does the same because he is the sole provider for an ailing parent. (According to WPO, one in four employees is caring for an older or ailing adult in some regard.)