This is the pattern I use to make DD's boutique outfits. For the skirt I just use two 42" lengths of the fabric. For the ruffle on the bottom of the skirt I used three 42" lengths of the fabric. The skirt is generally 12-15" and the ruffle is 3 1/2". Then I add a one or two rows of ruffles to the bottom of the pants....both 42" in length. The bottom ruffle layer is 4 1/2" and the top layer is 3 1/2"...unless I do one row of ruffles then it is generally 4".
For baby girl I just use a baby doll pattern for the top because she is so tiny! I hope that all makes sense!