Like the endless permutations of "hardcore/casual/casually hardcore" guilds, family-friendly guilds come in different flavors, too. Some are designed to be more flexible for members who are parents or who work demanding schedules -- but they're still essentially guilds for adults. Others are family-friendly in the sense that they welcome players of all ages. Some are focused on hardcore content under a relatively forgiving schedule and attendance policies, while others are casual to the point of becoming a glorified chat channel. Pinpointing what you mean by "family-friendly," what they mean by "family-friendly" and what type of content they aim to do in a "family-friendly" way is the key to finding a good fit.
For career types: Let's face it: Raiding doesn't always mix well with long commutes through bumper-to-bumper traffic, dragging in an hour and a half past raid time because the meeting ran late, or AFKing because the boss just called or you need to kick off the next report. Sometimes, you find you're technically able to make raid times, but you find yourself logging in with completely depleted health and energy bars after a long day at the office. Or you may need to log out for some shut-eye (and that 5 a.m. wake-up call) right as everyone finally gets the strategy down and is ready for another attempt. A family-friendly guild is more likely to keep reasonable hours, excuse work conflicts and hold those late-night attempts for another session.
For non-traditional schedules: If your work schedule is so wacky that you can't find a fitting guild in any time zone, a family-friendly guild offers an understanding group of friends who'll take the time to learn why you disappear for two weeks every month or suddenly start logging in just as everyone's winding down for the evening -- and not kick you from the guild because of it.
For parents: Family-friendly guilds are the obvious choice for gaming parents of young children. Not only will you receive blessed forgiveness for AFKing over a diaper change or temper tantrum, but the camaraderie among those going through the same issues will buoy your drained mana bar.
For families: Gaming is a great way for extended families not only to stay in touch but to enjoy a common pastime. Still, Grampa or your 9-year-old cousin may not be the swiftest player on the block. In a family-friendly guild, you're more likely to find your family members embraced for their charming personalities than shunned for their technical foibles.
For kids: For many guilds, game time means time to let it all hang out: raunchy jokes, colorful language, pent-up rage, you name it. That doesn't mean you want your kids in on the action. Remember, a guild may be accommodating to members who are parents while not being at all an appropriate guild for kids. If you want to play along with your children, or if you're looking for a place they can tuck in without being exposed to rough language, a family-friendly guild that encourages kids to play is the ticket.
The Number One tip for finding a good fit with a family-friendly guild: Besides asking what about a guild makes them family-friendly (their schedule? their AFK and attendance policies? the age of their members?), find out what their in-game goals are. If you're looking for a group that won't penalize you for the occasional baby wake-up in the middle of a raid, you're looking for a different guild than the mom who wants a hangout for her two grade-schoolers, Grandma and her nephews from college. Ask!