The thing you have to remember about "frontier" dress is that folks on the frontier never had closets. They had pegs to keep their clothing on, because they usually only owned two or perhaps three outfits; maybe one kept for "best" and the other two rotated each day and aired out in between. (Very seldom actually washed -- natural dyes didn't hold up well to being regularly scrubbed and doused in lye soap.) What frontier people laundered were their underclothes and aprons, which were worn in layers precisely because they were meant to protect the clothing from getting soiled.
Women and children ALWAYS wore aprons during everyday situations, and the pinafore aprons on kids were quite voluminous and covered more of the clothing than they revealed. Use this to your advantage. Get a good apron and it really won't matter what you put under it, as long as it has a longish skirt. (Use one of yours and pin the waist; that works fine.)
Here's the thing. A tabard-style pinafore does not really require more than 5-minutes sewing, and it was a style commonly worn by kids of the era because it was quick to make and didn't require much fabric. Just get a piece of plain unbleached opaque cotton sufficiently long enough to reach from your child's neck to mid-calf times 2. Buy a little bottle of FrayChek if you don't want to finish the edges, and also enough ribbon or bias tape to go around the child's waist 4X. Measure the child's shoulder width and add 10 inches, then cut the piece that wide lengthwise (add two more inches of width if you're going to finish the edges.) Fold it in half and cut out a square neckline. Now put the piece over the child's head and mark all the sides where the waist hits. Take it off the child, cut the ribbon or bias tape into 4 equal pieces, and sew one piece to each of the marked waist points. Either sew down the edges or coat them with Fray-chek so that they won't ravel, and you're done.
To wear it, put it over the child's head and tie the ties attached to the back at the front of the waist UNDER the front flap of the pinafore. Then tie the ties attached to the front flap at the back.