I believe kids need to wear undershirts in the wintertime. But I have lanky kids. I bought Bauer the smallest undershirts I could find and they are of appropriate length, however I can grab a fist-full of fabric on either side, and the collar can almost hang off of his shoulder. They do not fit too neatly under his clothes.
He does have a 3 pack of Carter's 18mo onesies from last winter that have perfect arm length and collar and waist circumference, and without diaper bulk, were even long enough. Problem being that they're onesies and he can barely get up on the toilet by himself, let alone finagle crotch fastenings himself. So I cut the snaps off.
I don't think it *needed* hemming since knits don't tend to fray and unravel, but I decided that I'd prefer it to look a tad more finished and also, I haven't used my sewing machine in months and needed to prove to myself that I could actually still operate it. I can.