Disneyland is not in L.A., so you need to add time for that.
It's about 9 hours to drive from L.A. or from Anaheim (where Disneyland is located) to the South Rim of the Grand Canyon. Spending a night is the best way to experience this lovely NP ~ sunset AND sunrise. Then it's 5 hrs to Las Vegas.
It is 'only' 5 hours to drive from L.A. to Las Vegas. Then it's 5 hrs to the Grand Canyon. Then it's 5 hrs back to Las Vegas. My route is an hour shorter and it doesn't backtrack, but it does have a long drive one day.
If you don't want/need to see GCNP this time, or if you don't mind renting another car to drive yourselves to GCNP, or if you have lots of money for the bus tour, or if you have tons of money for the fixed wing flight, then fly from a southern California airport (BUR, ONT, SNA, or maybe LAX) to LAS. It doesn't seem like it would save time, but it might.