I couldn't find any background info on how that scene was done. But notice that when we actually see them lowering the mice into their mouths, it's a different camera shot from the moment before when they picked up the moving mice from the container thing. After one shot is finished and the camera rolls again for the next shot, any amount of time can pass, to allow for all kinds of stuff to be rearranged and switched around. That includes changing props that actors are holding.
In the shot that shows the actors actually putting the "mice" in their mouths, the legs aren't moving anymore.
Kor