Working with kids can be a lot of fun, but it does require patience. Young kids will not always cooperate so it can take a few shots to get the one, and sometimes it may even take more than that, but when you finally get the perfect shot you'll see that it's always worth it. Kids tend to be very energetic which means you have to be really quick when it comes to composing and taking your shot.
There are some ways to utilize their energy. Raise your shutter speed and take some candid photos of them just having fun and being kids. Don't try to get the perfect pose every time. Sometimes the best photos are the ones where your subjects are just being themselves, that goes for adults and children.
I've found that a lot of times children won't put on the smile that you'd want them to so it always helps to have an assistant there to help you make funny faces or act like a bird. In my case, I have Kenia, who is just so great with kids, and loves to make them laugh.
This one was short, but if there is anything to take away from it it's to be patient and have fun. Your clients chose you to capture memories that will last forever and the least you can do is give them your time and enjoy every second of it. That is what you are there for to begin with.