Easiest way? Copy it from a shirt or dress you already like and that fits well. If you lay the clothing out really flat, you can actually place a piece of typing paper over the armpit area and use a pencil to draw the curve, feeling where the seam sticks out a little. You'll probably need to draw a couple of different armscyes, say one from a T-shirt, one from a fitted top, and one from a dress, because we like our armpits to fit differently depending on the item.
Harder way? Get a piece of wire or bendable ruler that will hold its curve and bend it around your armpit, then trace that curve.
Then make something goofy for your kid:
In other news, this is unrelated to anything and is also pretty offensive, BTW, but I cannot get over the awesomeness that is Why the Frak Do You Have a Kid? It's really bad, like snapshots of groups of smiling pregnant fourteen-year-olds in prom dresses and goth guys holding babies while playing video games, etc.--you know how much I love parenting train wrecks (Hello, Toddlers and Tiaras!). Check out the entry for April 3--I started screaming out loud and basically could not stop for a Really. Long. Time.
It got to the point in which I was drinking a glass of ice water, and I'd have to consciously take a drink, swallow, and then put the glass to the side before I pulled up a new photo.
I'm such a cliche.