How to adjust sizes

My son is a skinny 5 year old. I'm wanting to make the art museum trouser and he has a 19 inch waist and 22 in hip with a height of 40 inches. I'm guessing I should make the 2T size, but where would be the best place to lengthen the pants? Should I cut out the 2T size and just add an extra few inches at the bottom hem?