You could always take that pattern, double-strand the yarn and go up several needle sizes.
Something to consider is starting with a provisional cast-on, so that as the child grows, you can lengthen it (buy extra yarn). It's not nearly as difficult to unpick a BO as it is a CO edge.
But looking at that pattern, the circumference of the bottom edge is 51 inches... that would almost fit me.
What I would probably do with that is go up a needle size (or two), use a provisional CO, and start with round 4. Once you have the decreases, shoulder and neck done, and you know that that portion fits your LO, go back to the CO edge and work the length you need, then add rounds 3, 2, 1 and BO.