1. Use a fast shutter speed. You'll want to shoot in manual mode to make sure you capture their fast action. Or Shutter Priority mode works until you get more comfortable with your camera.
2. Temper your expectations. Arrive at the shoot with an ability to "just roll with it". Toddlers are often unpredictable and very often, the photo shoot is on THEIR terms! But that's what makes them fun. Your job is to capture this part of their personality. That's why they are one of my favorite subjects! While you want to deliver "posed" shots with everyone looking at the camera, know that those shots will take more work on your part, and likely require you to be in a continuous shooting mode on your camera.
3. Eyes on YOU. I often see parents and siblings trying to tell a toddler to "look at the camera" during a shoot. I say, tell the parents to focus on looking at you, because when the split second comes that kids are looking at the camera, you don't want parents looking down at them!