Here are a few example's of some I have made:
These are from That*Darn*Kat and they are uber easy. They are a little more full coverage but I love that. You can find her pattern for sale on Etsy. It comes with both the boy and girl versions. If you have a girl you will be done in less than 30 minutes, I swear. They are so quick.
These are from Fishsticks Designs. Please go look at all of Bonnie's patterns. I have a slew of them and they are fantastic. These will take you a little longer but you will love the results. They come out beautiful! My son is almost four and runs around the house half naked most if the time so these tend. To be a bit more fun to run around in.
Last but not least are these really cute undies from Elysium. She also has an Etsy shop. These are unisex as well so they a bit more low cut in the boy version. They are still quick to sew up and come out extremely adorable.
I hope you love all of these and give some of them a try. Underwear for the little ones are very fun and satisfying to make and make it so easy to feel as if you accomplished something.
