First lay down a loose fitting shirt of your kiddos to get a close size. You want to pick a top that doesn’t need to stretch to be put on or worn. Cut the basic shape. For the neck I’d cut the V shallow since you can always make it deeper if necessary.
Cut points into the bottom edge of the shirt pieces. The front and the back are the same.
Sew together at the shoulders. Then sew the side seams.
Try it on your kiddo. If it doesn’t fit over their head cut the V a bit wider in the front. Grab a scrap of contrasting fabric and tie around the waist for a sash. If you need it longer then tie sew the edges of two straps together. I didn’t finish it at all.