Use a tripod and remote
Assuming you’re using a DSLR or point-and-shoot rather than camera-phone, you can always set up your camera on a tripod and use the remote to surreptitiously take photos when the kids are least expecting it.
Give it time
Usually even camera-aware kids get bored of making faces after a while and stop paying attention. Adults are blessed with a longer attention-span and sense of determination than children, so just wait it out. Take photos all the while, so the kids get used to the camera.
Take lots of photos
Although snapping a ton of photos like a tourist may seem sort of tacky, get the kids used to the camera by snapping as many photos as possible. That way they may stop pulling faces and instead “say cheese” for the camera.
Learn to love funny faces
Embrace the pulled faces! It’s who they are now, and that’s what you’re trying to capture.
Give your kids a camera
There are durable digital cameras made with young children in mind. It stand to reason that if your kids get to take some of the pictures they might be more cooperative!