I don't think it's possible to tiger-parent K-aged kids into doing 2nd grade math. There are cognitive development issues that can't be gotten around. It sounds to me like your kids are definitely gifted. (And the quirkiness thing is a bit of a red herring -- some gifted kids appear quite normal!)

Rather than testing to find out if they're gifted, I would save testing for if you need scores to convince the school to place them where they will get an appropriate challenge.