for my daughter (5) I used a worsted weight wool, and went to a size 10 needle. it worked PERFECTLY with no pattern changes! it is roomy enough for at least another year or so. I think many have done the same. using the worsted weight instead of the bulky I think is the key. that way no extra math is involved. (LOL) you may want to swatch with both an 11 and a 10 to see which fabric you like better with your yarn.
I love this pattern and have made several- it works up really quickly! let me know how it works!