I knitted a Weasley sweater for my baby, and I just had to post a picture here.
I used
this pattern for seamless in-the-round raglan sweaters and scaled it down to baby size. One thing I learned is that the percentages on that pattern for the sleeve width don't work for baby arms -- they need to be a lot wider! I'd hoped the sweater would fit him through the winter, but the arms are already tight.
I used a soft acrylic yarn (something cheap from Michael's in case it didn't work out), but I was really pleased with the result. The letter was duplicate stitched on, and the pattern for that was adapted from the letter pattern in Magical Knits.
Also, you can't see it here, but I divided the neck about two inches down from where the collar ended and created a tab with two buttonholes for a better fit around the neck.
I was really proud of the result! :-)