I have always done some version of this, actually.

I sometimes had Silverback inside with as many as 4 bitches.
Poncho could play with a whole litterful of pups ... he'd lay on his back and let them tear his face apart, ripping his ears, making him bleed, he didn't care and didn't feel it. Was very gentle with pups.

Like everyone, I have had dogs that just "couldn't see hair move" without going ballistic ... but my BEST dogs have always had stable dispositions ... and have the sense to recognize the difference between friend and foe.

Jack