This is a question I get asked a lot! It is easier than you think. The first thing I suggest is to play with your kids and make sure they have fun, this will give you beautiful pictures since you do not have to ask your kids to say: "cheese" and capture a fake smile.
You want to make sure when you look at that picture in 10 years, you will remember how much fun both you and your kids were having; you want to remember how they were cracking up because of a silly word you said. I call this great photographic memories.
The only other thing you have to do is make sure you are ready to take the picture by holding the camera steady and squeezing the shutter button gently. Only this way you will capture a beautiful tack sharp portrait of your kids.
Easy right? So, grab your point and shoot, your DSLR, your camera phone, whatever you are most comfortable with and have fun with your kids; sooner than you know they will be off to great things and spending time with them might be a hard thing to do.