So, I'm an admin. asst. and I have to take up the slack of the woman I work with when she is out on her 2.5 week vacations to exocitc places with her husband. She has no kids. I do the same when she's out sick, or she has to when I'm out for one reason or another. And guess what? Sometimes I take a "sick" day when I'm not really sick - maybe I have an audition, or my wedding dress fitting, or I just want to do laundry!
If you have personal/sick/vacation days you can use them. And unless your company policy states you must have doctor notes for sick days and plane receipts for vacation days, you can use them however you want. Part of my job is covering for people who are out - no matter the reason. I think people give working parents a harder time than others because they know the sick day is really for their kid. And that's not fair in my book. As long as they aren't abusing policy you can't really bitch about it - unless you also bitch about covering for Suzy Single when she's recovering from her boob job.
BTW, I'm a pretty tried and true feminist...