Kid and Law are newcomers?
They've been a part of the story for 14 years now and have had multiple scenes where they stood side by side with Luffy. It wouldn't be farfetched to have them beat Big Mom (or have her retreat), especially in a 2v1 situation where captains are fighting another captain. You'll get what you're asking for with the admirals as they're more than likely reserved to be the Monster Trio's opponents.
Literally, nothing is going to change if Kid and Law win.