Here's how you can make one of these asymmetrical rainbow button sweater cardigans for your kid: (and since this is my second tutorial ever, it's a really really good one.)
Get an old women's cardigan, solid color works best! Grab a currently fits your kid well cardigan to use as a "pattern" - and I use the term loosely.
Cut the big cardigan into peices (make sure the buttons are all buttoned, on both sweaters) - front, back, and two sleeves.
Cut the back peice. Place the kid cardi on the back peice, line up the neckline, fold in the sleeves, and cut. Allow some extra for the seam allowance.
You'll end up with this:
Now, do the same for the front peice, but don't line up the kid cardi straight up and down. Notice how the big cardi button placket goes off to one side of the kid cardi. Then cut. If you would prefer your placket to be front and center, then line it up that way.
You should now have front and back bodice peices.
Set your machine to zigzag OR... if you have the following, it will save you effort: It sews a reinforced seam along with a binding to keep the material from unraveling. If you don't then use a zigzag on the edge and then a straight seam. Or straight seam and then zigzag whatever you prefer.
Flip right sides together, sew the top shoulder seams. You should have this:
Now cut the sleeves. Lay the kid cardi sleeve on top of the big cardi sleeve, and cut along leaving a seam allowance.
Pin the sleeve to the sweater around the top of the shoulder, sew. Do this to both sleeves.
Pin along the bottom of sleeve, and along side of sweater. Sew. (note, you're still sewing wrong side out). Do this to both sides.
Hem along the bottom of the sweater. Now you've got a functional asymmetrical little kid cardi.
Last step, sew new buttonholes inbetween the existing buttonholes (you'll need more holes to make a whole rainbow!). Remove old buttons, sew a rainbow of new buttons down the placket. And you're done! Good job.