I have not seen this pattern yet , but in the past I have converted tops to rugby’s by the use of inch wide cotton webbing tape . Leave the front pattern plain and re cut your slash off centre by 3/4 ”( other wise your buttons won’t be centered
You would omit the front facing, attach the tape to the front placket with your machine foot far right ( as if you are attaching a zipper) right sides together . Sew with a tiny stitch ( stitch length 2 or less).
Press to shape, so you have a bottom placket ( the webbing ) and a top placket ( the top with webbing underneath)
Press webbing placket seam to the top and top stitch, stopping at the bottom of the placket
Press the placket flat and topstitch all layers at the base of the placket.
Check this goes over the child’s head before attaching the collar, if it is too firm you can trim a little from the neck allowance and add that to your collar.
I then attach the collar to the inside , so your seams are exposed on the outside
I cover the seam with cotton webbing, not as wide as the placket , but a good width to cover the raw edges
Have a look at a purchased rugby for different finishes
Clear as mud ?