It's not uncommon for a project to change gauge midstream. You become more relaxed as you knit. I have heard that you should knit for awhile on something..anything to get into the rhythm of knitting then knit the swatch.
I had the same thing happen to me on a sweater I made.. I had to rip out 5 inches of the bottom of the sweater or it was going to be big enough to fit Hagrid. Pain in the neck, but worth it in the end.
Now, with a kids sweater I probably would just continue because kids grow and it's better to have it a little larger than too small. If you don't want to do that I guess you could change needle size, but that may be obvious depending on the yarn and how much difference there actually is. Frogging is the other option.
When asking questions ALWAYS post the name and a link for the pattern if you have it.