For this pattern, use any DK acrylic yarn and a 4,5 mm size hook (or whichever one you need to reach gauge).
 
This lovely sweater is designed with a positive ease, so your fast-growing little one will be able to wear it for a while to come. Did your cutie just reach the age category or would you like to make a directly-fitting sweater, then opt for one size smaller. The sizing is given for: 2-3 yo, 4-5 yo, 6-7 yo, 8-9 yo, 10 yo