This is a common question asked by people sewing gifts for others. Even WITH a childs measurements a lot of the ‘Big 3’ or ‘Big 4’ like Simplicity, McCalls & Butterick that you can purchase in stores the sizing is just way off. I personally have found that the sizing to measurements on European patterns seem to fit much better. But there are lots of great cute ‘Big 3’ patterns so here is the best way to decide on a size for the special little child!
First off find out what they RTW size is (in store size) Then either hit up a garage sale, local thrift store or your local freecycle for a shirt (non-stretchy) a pair of pants & a dress (for girls) to use as a measuring device!!!
Lay the choosen item on the pattern piece you are trying to determine the size for… you want it to have about 5/8″ (or what ever the seam allowance is) on each side… make the size that best fits that! Remember if they have an elastic waist to stretch it out. You will find that the patterns tend to most commonly run wide… so if you have skinny kiddos you may need to size down, in that case just add some length to the pattern piece!
Hope this helps answer some of those questions.