First, it's much easier if you have a good profile photo. Try to get your little one in front of a plain white wall, but if you have an 18-month old like myself, you just have to work with what you can get. Here is Harvey being entertained by one of the girls in front of a not-so-ideal background. (I know, I REALLY need to trim the back of his hair.)
Open your photo, and choose the magnetic lasso tool. Take your time and pay close attention to the details. The best silhouettes have small details like smiles, hair wisps, and eyelashes. Click to make sure it sticks, and you can cut off anything undesirable (like the hair fluff on the back of Harvey's head.) Oh, and you can hit the backspace button to "un-magnetize" the dots one by one if it sticks somewhere you don't want it.
When you're happy with it select the inverse...
... duplicate your layer, add a blank layer between the two, and (make sure your top photo layer is selected) hit the delete button to get rid of the background. Turn off the bottom layer by pressing the little eyeball next to your photo in the layers palette. Now you have a floating head. Use the eraser to get rid of anything extra, and use the brush to add a little to the neck if you need it. Keep in mind, that the whole picture will be black so it doesn't matter what the picture looks like as long as the outline looks good.
You're almost done! All you have to do is turn your picture black. Choose the Hue/Saturation, and move the darkness/lightness slider all the way to dark and your photo will be black.
I also added his name and the year so we could see how he changes through the years. You can print your silhouette on white paper and frame it for a simple and sweet Mother's Day gift.