sew them together using a 3/8″ seam allowance in straight stitch (make sure to not sew the extra red/grey strips into the seam). iron out the seams toward the blue fabric. then top-stitch the edge of the blue fabric 1/8″ away from the edge. complete this step for the back of the shirt.
you may want to check the fit of the sleeves and shirt length on your child before the next couple of steps. you will be hemming them and will need an extra 3/4″ from where you want them to be. cut off any extra fabric. hem the sleeve edges by folding 1/4″ pressing with iron then folding another 