Fisting is actually quite the art and quite an amazing and erotic experience. I have been lucky enough to have had my hand inside a woman. Both parties need to be into it and want to do it. Whatever anyone else says, fisting can be done, regardless if she's had children or not. How do I know? Because my ex certainly hasn't had any children. If she works her Kegals well, she can stretch the area and yet remain tight enough for your cock. It's one of the hottest and most erotic things I have ever done, besides being with a squirter (which my ex was as well, ha!). And believe me, there's a heck of a lot more to it than just making a fist and trying to slide it in. When my hand got inside, it was after 20 min of work via fingering, licking, playing, and lube, lube, lube. And when inside, the orgasms to insue were unforgettable to witness and feel.
I can barely get two fingers inside my girlfriend. Before we started having sex she used to say putting a tampon in would sometimes hurt. I think given those two facts that trying to fist her might be a bad idea.