A lot depends on the driver and the type of food you are eating. I have seen drivers refuse people with ice cream cones, cups with no lids, and a watermelon slice. Uh, yeah. I wonder why. ::::::::::said very sarcastically:::::::: Oh, and a can of Cheese Wiz and sleeve of Saltines. There's a whole story behind that one...
The juice box won't be a problem, but a muffin not in a baggie very likely might be.
A lot of times you have to stand. And with the corners those busses take, there is no way you are going to stand holding on to a juice box and muffin, without someone wearing it.
Please don't take offense to what I'm going to say next--it's meant to help you enjoy your trip, not criticize...
It only takes a minute to eat a muffin and finish a juice box. Why not just make life easier and finish breakfast before leaving your room? There is nothing in a theme park that won't be there if you take the next bus. Trust me. I've been going to WDW for 22 years. I've learned that "Commando Style Touring" wears you out, you miss some of the best things, and end up with indigestion.
Anne