I have found that TCP pants/shorts are all too big in the waist for my kids. I hope they start making the adjustable waist for babies/toddlers soon. My older two kids were wearing 18-24 months the season before they turned two. Now, my second youngest will be three in September and is still wearing 12 month shorts. I sure wish I was that thin!!!!
My youngest is also 11 months, and is currently wearing 9-12 month clothing. I just shopped at TCP at the outlet near our beach, and got him some 18 month shorts for next summer. As I said, the waist always seems huge, and since there isn't a length issue with shorts, that seems safest. Now with shirts, I would go with a 2T/24 months, since they can be a little big with no problem. All of mine have been in the tiny side, though (no more than the 25th percentile).