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.