Tooky94
The ONLY way to even try to work with the four biggie pattern companies is to measure the child. Use only the measurements, and not the RTW size. Those numbers mean nothing in patterns. When I do custom work, I measure the chest, waist, hip (if necessary), arm girth, back waist length*, and final desired garment length.
*The back waist length is a tricky measurement, but very important -it helps for making patterns for string beans.
Put a necklace around the child's neck and let it fall naturally. This is the exact spot a dress should end. Secondly, safety pin a ribbon around the child's waist. This one can be tricky on the little square bodies. But somebody once said to me, "Oh, you mean that space right before it curves down to her bottom?" Well, yes. Then measure the child's BACK from the necklace to the waist ribbon. (It doesn't matter how long the necklace is in front) That is the back waist length. This number is also referenced on the pattern measurements.
Once you have your measurements in order, compare them to the ones on the pattern envelope. Don't let it scare you that the child you are sewing for wears an 8 RTW, but needs a 5 pattern.