You would need to completely redraft the pattern. The largest size is 20" chest. If, say, you now need a 24", then you need to add an inch each side of the front and each side of the back. Do a test square to see how many stitches you knit to the inch, then add this number each side, front and back. Bear in mind you will also need to make it longer, but that should be relatively easy to calculate by just measuring the child.
You will also have to work a few extra stitches into the neck shaping. Probably best if you draw the whole thing on a piece of graph paper (but remembering, of course, that knitted stitches aren't exactly square, so the finished draft on graph paper won't necessarily like in proportion).
And then, of course, you will have to adjust the sleeves accordingly.
Of course, you could always take a chance and knit the whole thing using, say, Aran yarn or even chunky knit with the appropriate size needles. Just follow the pattern exactly, adjusting the length where necessary.