There is also another pattern on that site for a woman's jumper, that has an xs size, that maybe you could just adapt it to one or two sizes smaller
Here's one that has a size 10
You could start this one at size 14 and make this one longer and decreasing until it has the amount of stitches needed for the size 10, then continue the top part following the size 10 pattern.
You could even knit a short tank top, then turn it upside down and pick up the bottom edge of the top all the way around and either increase by 1/3 the amount of stitches evenly to in the next row and work even until last 2" and work garter stitch or a dexign on the end, or increase a few stitches until you increase by 1/3 and finish off.
I've been looking for a jumper pattern that I like for my 3yo. I'm going to combine 2 patterns to get the look I want.
Good luck to you.