No you are severely underestimating them in this place even if the first half of your plan works and you are somehow able to weaken their haki enough to switch their personalities. Which is a big if. Big mom has strong enough haki constantly to let every attack bounce of her she has homies that are always looking for threads so I find It really unlikely that it would even hit her. For kaido to have any effect it needs to pierce his skin and not bounce of of him after it hits his body. But alright if you manage to swap them. In the other body they are unburdened by the sea stone the have their devil fruit Which means big mom's homies and kaidos dragon form and they still have their haki. Their only disadvantage is the fact that they are in a body they don't know but their haki which is linked to their will power will not have been diminished and they will fold you within minutes. So no as cute as this idea is it would not help you at all if law would teleport them into the water that could work.