I think all of Jen's suggestions are right on target. You are right, it isn't a good idea to share your mixed emotions with the kids as then it confuses them. I know how hard it is.. as both a parent and a teacher. Although your children are looking to you as their guide about this very new and exciting experience.
If they sense that you may have mixed emotions about things and may not be confident about leaving them at school, that sends the signal that this may not be a good place for them to be and they may have trouble separating.
I tell my parents to try and be as positive as possible and do those things Jen suggested. Keep a big smile on your face when you drop them off.. tell them what fun things you will do with them when you get home.. a kiss and hug and goodbye. (then you can cry when you get around the corner in your car.. ha.!) I know.. I have been through leaving kids at college too.. so hard I know! My "baby" will be leaving for college next fall. THAT is going to be so hard!
Short and sweet is always best. Reassure them that mommy always comes back.. (or dad or whoever is going to pick them up) Expect some tears if they haven't left you before but it will get better. And also if you haven't left them at a class or something before or with a sitter or family member for short periods of time. do try and do that before school starts so they know you will always return and build some confidence with this.
There are also great articles in parenting magazines (you can check things out at the library and google the topic online..)
on this topic and separation anxiety for both parents and kids..It seems now a days, children need at least one year of preschool to help prepare for kindergarten so you are definitely doing the right thing.
hope this helps.. I'm sure they will be just fine! I think its wonderful that you love the kids and will miss them. But just reassure yourself also that you know this is a wonderful experience for them and it will give you some time for yourself too!