Not long ago, a friend was telling me that her 3-year old was desperate for a pair of Snow White underwear, but no matter where she looked she couldn't find them. She couldn't find iron-on transfers, homemade undies on the internet, or patches small enough....so I volunteered to try to make her some. She let her daughter pick out the fabric to ensure it was what she wanted. I was only able to find two helpful, free patterns online. This one and this one. Though they are both very similar, they both have their pros and cons. The second pattern only requires 2 pieces of fabric, which was nice, but the Princesses were upside down on the back. Upside down princesses on the butt don't bother the 3 year old, so I'm in the clear, but I feel it's worth noting.
Once I find out if they fit the child well, and/or if she preferred one style over the other, I will make her more and/or make applique's out of the fabric and sew them onto plain undies (kind of like what you see to the left in this photo - that was an experiment).
I found some underwear patterns on sale on etsy.com that I really liked, but they use all knit fabric, which wouldn't work for this particular project. I think if my desire to make underwear for my own children was a little greater, I'd buy one of the patterns. As it stands right this moment, I am perfectly content buying their underwear