You can do a swatch, but if you don't want to the Knitting Police won't show up at your door. Since it's a toddler sweater, and the kids change size, exact fit may not be an issue.
I wouldn't try to knit tighter because you'd have to be conscious of knitting tighter all the time. If your mind wanders and you relax a bit, your going to have varying tension. I'd go with the smaller needles.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry