First off, plan your location. If you're going out, make sure you keep to a small space so you won't need to chase the dog for miles. Stick to one location. If you move around too much, you're opening up a can of distractions right there. If you're doing it in a room set-up, keep it interesting for your models with props and activities.
Have fun! You can't command a single pose and stick to it all throughout. You can't bring that kind of stress in this kind of situation. Just have an objective in mind and keep your mind open on how to achieve it. If you're going for an emotional connection between the boy and the dog, then make sure they're the only ones in the area. Let go, let them do their thing, and have fun watching them.
Be PATIENT, be ready. Just let your models loose to interact with one another and you'll catch wonderful nuggets --as long as you're willing to wait and as long as you're quick on the draw. Major pet and kid photography rule: Natural is always better than staged. Kids and dogs should be kept motivated, not dictated. Simply keep your finger on the cam trigger for these beautiful stolen moments. Your patience will be rewarded, I guarantee you.
Manage the time. When you've been shooting for 3 hours, it's only normal for your young models to get tired and cranky. Think. One of them is a child who wants to have fun, and the other is a dog who doesn't care about your little project. For your sanity, make it short and sweet.
Follow these and you might even end up being a better photographer than the pros. I mean, you're the only photographer who'll have milk, cookies, a movie, and a bedtime story later for your models, right? Definitely a plus! :)