Basic facts 3 kids in someone else's pole barn, playing with lighters, burning small amounts of hay, and baling twine, maybe a shovel handle. One acount states the burning twine placed in a hay bale, one account burning twine dropped "accidently". Fire starts attemts are made to extigusih, it goes out of control burns structure to the ground.

Can or should I charge both? Criminal Mischief seems easier wtih "knowing" mental state. Both 2nd degree felonys

MHO, I think you're better off with a criminal mischief. With your facts, your problem is going to be "with intent" to burn a building.
Even though there is reckless language in the arson code now, a good defense is going to hammer at the kid's intent was to burn the twine not the hay/building. I had 2 kids once burn some twigs and hay that then set aflame 47 round bales and a volunteer fireman get killed by turning over a pumper truck on his way to the fire. It was a struggle from start to finish ! You are a little better off in your case since they did burn a building. I had to call in an expert from A&M to prove whether hay was vegetation.

Not that I've had any experience with this, so take this for what it's worth, but when I read your facts my gut said that a jury would probably convict on a criminal mischief ("mischief" sounds like what the kids were doing) than an arson (not really what laypeople think of as arson, now, is it?). Yes, technically, it meets the definition, but I would think it would be harder to convince a jury to give them a criminal record that says "arson" for those facts. Criminal mischief seems somehow easier to swallow.

I vote criminal mischief. Of course, I would never disagree with Stacey.