I have trouble seeing Kid and Law defeat a Yonko before Luffy, Oda is way too bias for Luffy. Oda probably wants to have Luffy defeat a Yonko first and make a big deal out of it, that's why I have trouble seeing Big Mom get defeated before Kaido.
The only other four possibilities I can see happen are:
1. Big Mom gets defeated after Kaido(I'm not too sure if that will happen either)
2. Big Mom gets defeated at the same time as Kaido
3. Instead of defeating Big Mom, Kid and Law remove Big Mom from Onigashima
4. Big Mom defeats Kid and Law again and then leaves Wano