Even for women's clothing, I've found significant variation between brands. But in the children's clothing I buy, sizing is all over the map!!
In shopping for my boys (aged 7 and 9) I've found that the smaller brands tend to run on the smaller side. Gap Kids is long in the legs, good in the arm length. Sports brands like Nike and Reebok are right on, in my opinion. TCP is inconsistent in tops, but pretty good in bottoms. Old Navy has huge sizing variations from one pair of jeans to the next!
Are the cheaper brands trusting their pattern grading to overseas factories' "standard US sizing"?
Sorry, but I'd rather do my own grading. Then, even if my customers don't like my sizing, they can at least know what to expect from one collection to the next.