As long as you're keeping to the 6 row pattern and placing the buttonholes with 13 rows between them (or close to that) you should be fine. The wonderful thing about knitting for babies is that they look adorable in everything and they never complain about the knitting. Post a picture of the finished sweater if you'd like. It would be fun to see it. (Thanks for the pattern photo.)
Congratulations on the expected baby!