I seem to favor knitting in the round - weave in loose ends and the garment is wearable right away.
This pattern is one of those as well :)
The shirt is knitted from the top down, and the pattern comes in the following sizes:

6-12 months (17-19 inch chest)
1-2 years (19-21 inch chest)
2-3 years (21-23 inch chest)
3-5 years (23-25 inch chest)
5-7 years (25-27 inch chest)
7-9 years (27-29 inch chest)

By popular request I added two adult sizes to the pattern.

Women Sizes:
32”
34”