Can any of you give me any hints on how to get up the stairs in time to open the castle doors? I tried splitting the babies from Mario & Luigi and leaving the babies at the top of the stairs, but the babies can't jump high enough to hit the ? mark.

Also, how do you jump high? I can't get up to get beans on high platform in Peaches' castle.

You'll learn new moves from Toadsworth later on in the game, don't worry about it right now, there's no way to get up there before that.
For the stairs in the castle you need to be able to roll into a ball, which makes you go much faster.
For the high places with beans you should be able to nitice when you learn that one, it makes the babies go higher and fly.

Good luck