One product that I always use when sewing doll clothes with knit fabrics is wash away sticky tape. It's an absolute must for me and I will not sew knit doll clothes without it. Especially when clothes open at the back, the center back seam will stretch outwards and that annoys me no end....

The photo shows the Dritz brandname sticky tape, Wonder Tape, that I use. This brandname version is pricey and a roll dwindles fast. At the same time there are many generic versions of this same material available. Those are MUCH larger rolls and economical in use. However, my sewing machines are expensive and so I personally won't use the generic version of this material because if I gum-up my machine I'm out of a ton more money than the cheaper tape saved me and it's not worth it for me. Something to think about.

Here I use Wonder Tape along the center back of a small T-Shirt. It's very easy to use, you simply tape it down, along the edge and remove the paper backing.

Next you fold it over to the right side,  press to secure. And you can stitch the edge down. I always use a zigzag for that.

As you can see, the seam stays completely flat and there is no chance for this without the tape.

Have a great day with lots of sewing!