Its about two kids who run away. They are brother and sister. They pack all of their clothes in their orchestra cases. Then when they get on the school bus they hide in the back of the bus. When it gets to the bus barn, they get off and go and stay in this big bed in a museum. And that's all I remember.
Anyone know what movie I'm talking about?

Could be From the Mixed-Up Files of Mrs. Basil E. Frankweiler.

YES! That's IT! Thank you so much! I have been wondering what movie it was for years! Once again thank you!