Woes of Motherhood

My children just started school, like many others, the beginning of this month. My son, age 6, now in first grade and my daughter in Pre-K. My son is doing fine. School is like an old hat for him now. He has friends in his class, seems to like the teacher, and deals with the bus. My daughter on the other hand is not coping well, at all.
So where do I begin? She just turned 4 yesterday, so she's still quite young. This would have raised a flag for me, but I know her all too well to think that she's not ready for school. This little darling found a Disney Learning activity book that I bought for my son three years ago, opened it one day, and I swear she hasn't put it down since. She went to school knowing how to spell her name, write her name, spell her brother's name and write almost all the letters of the alphabet. So, I would have to say...She's ready! It all started the first day of school....she was so upset on the bus ride home. She was hysterically crying when she got off the bus at the babysitter's house. This has not stopped. This is now the third week of school and the crying persists. It has now trickled into the school day. She has the day figured out and begins crying in the morning before we even take her to school. Then when daddy leaves she quickly calms down (thank heaven for her wonderful teacher). Then, the day goes mostly fine, until the last activity, that's when the panic attack hits. She starts crying and becomes hysterical. The only explanation of this from her is that the bus is bumpy, too long, and she has no friends. She has also said that she doesn't want to go to the babysitters, she wants to be home with mommy, and that the slide at the sitters is a baby slide.
I know, I know...this is a big step for her. School is an all new adventure and this means she is no longer a baby, which quite frankly worked out well for her. The only other changes are the kids at the sitter. She is the oldest now, her brother is only there for 20 mins with her just before we pick them up, there are three other kids there all under two.
So here is my question to all you mommy's and daddy's.....WHAT DO I DO? PLEASE HELP!