1. WORK WITH THE WEATHER. ANY TIME YOU SHOOT.
If you have a session outside, you need to treat the weather as one more subject in your shoot. That doesn't mean you avoid anything but perfectly warm and sunny days - based on where you live, that can grind your shooting days down to very few. I've shot in hot weather, rather cold weather, rainy weather, heavily cloudy weather - just know what you're walking into and prepare yourself and your subjects accordingly.
2. TREAT AN OUTDOOR SHOOT LIKE AN EVENT.
Whenever you plan for an event, you tend to plan for the beginning, middle and end of it, taking into account all the details. The same is true for an outdoor portrait session. Orchestrate the opening – where, exactly, will you meet, how will you start out, when should you plan for water or snack breaks, where will you change clothes as needed. A bit of upfront work, and good communication with the client, goes a long way.
3. START SHOOTING NEARLY IMMEDIATELY.
Unless you have an extraordinarily shy child or uncomfortable family dynamic, one of the best ways to get started is to simply start. I will chat a bit with my subjects when we first meet but, fairly quickly after that, I will start snapping candids. A lot of the looks and expressions you get in the beginning of the shoot won't show up later, so shooting early is a fun way to document a wider range of expressions.
4. BRING A SOURCE OF FILL.
Even though you'll be shooting outdoors with the greatest main light of all, the sun, that doesn't mean you will have the right kind of fill light you need to fill in the shadows caused by the main light. Bring along a simple reflector, a flash for fill, or whatever you need to be able to better illuminate your subject.
5. REMEMBER THAT IT'S ALL AN EXPERIENCE.
Part of the fun of shooting outside is a more varied experience. That's a good thing quite worth enjoying. Encourage your subjects to enjoy the experience as much as possible by making it as fun, meaningful, silly, or adventurous as possible. That sense of joy will definitely come through in your photographs.