1. Work with the weather EVERY 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 should avoid anything but perfectly sunny and warm days. Based on where you live, that could leave you very few days on which to shoot. I have shot in hot weather, cold weather, rainy weather, heavily cloudy weather...you name it! Know what you're walking into and prepare yourself (and your subjects) accordingly.
2. Treat an outdoor shoot like an event.
Whenever you prepare to host a big event, you make sure to create a plan for the beginning, middle, and end of the event; every detail and possibility is part of the overall plan. The same should be true for your outdoor photo session. Orchestrate the opening: where (exactly) you will meet, how you will start out, when you should break for water or a snack, where your subjects will change clothes. A bit of pre-shoot planning (and great communication with the client) goes a long way.
3. Start shooting (almost) immediately.
Unless you are notice an uncomfortable family dynamic or are working with an extraordinarily shy child, one of the best ways to start a shoot is simply...to start. I will chat a bit with my subjects when we first meet, but I will start snapping candids fairly quickly after that. 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 light.
Even though you'll be shooting outdoors with the greatest main light of all (the sun), that doesn't guarantee you'll have the right kind of fill light to fill in the shadows caused by the main light. Always bring a simple reflector, a flash for fill, or whatever else you might need to better illuminate your subject.
5. Remember that it's all an experience.
Part of the fun of shooting outside is a more exciting and varied experience. That's a good thing, and it's worth enjoying! Encourage your subjects to enjoy the experience as much as possibly by making the session fun, meaningful, silly, or adventurous. That sense of joy will definitely come through in your photographs.