Can I use interior paint on a tree house?

I'm sure the short answer is "no" ... but... this really doesn't need to be a high-quality paint job. I told the dude at the store what I was doing and after I had painted a small section of the tree house I realized it was interior paint. Is the paint going to make my wood chip, crack and explode or will it work fine but only for a couple years rather than long enough to serve my kids' kids? If I do take it back to the store and get the right stuff, can I paint over the section I've already done with exterior paint or is that a no-no?