1) Take advantage of the elements you can control and accept the ones you cannot. Rather than stressing out about the fact that your child's hands are stubbornly covering his face, focus on something else. If you have taken my class, you are starting to learn about things you can control with your camera, like depth of field. The environment you are taking pictures in is also up to your control. If you select an area to take pictures in with beautiful light and nice scenery the resulting image will be much more satisfying even if you child is making a silly face.
2) Don't spend too much time posing. Most of the time you try to pose children it comes across as unnatural anyway. Candid shots of kids having fun are great. Here is an example of a shot that combines the first two concepts. A beautiful scene and a kid having fun makes for a great shot.
This child was pretending to sing opera...lol, but he was doing it in some beautiful light.
3) If you are going to pose them, ask them to do something fun, like jumping into the air!
4)Take a traditional kind of portrait to some place more interesting.
5) Encourage your children to forget about the camera. This one can be especially difficult if you've been doing the opposite by teaching them to smile at the camera. This usually leads to a big ol' cheesy grin when the camera comes out :) I've seen it a BUNCH! Try taking pictures while your child is playing with friends. That's sure to lead to some spontaneous, genuine laughter. A great trick may be going out for some ice cream. You can sit together outside and snap pictures while your children are enjoying a treat together. Go close to sunset so the light will be soft.
4)Be patient. If you are desperately wanting your children to behave a certain way for the pictures they tend to pick up on this tension. Just relax and after all the silliness works it's way out, if you ask them to look at the camera, you may just get a strikingly genuine expression.