Preschool can be very difficult for young children. They don't have a concept of time, so it is hard for them to understand that they will be at the preschool for a portion of the day and then you will pick them up again. Kids can have a difficult time letting their mom or dad leave.
Everyone has seen scenes where a child is clinging to their parent, tears coursing down their face, and the parent is reluctant to leave, glancing around for some help. This is not only hard for the child, but also for the parent. I think we have all seen or faced this experience.
Situations like these can be less stressful if you follow a few simple ideas. If you are starting out at a new preschool, visit before the first day and take a tour of the school. Tell your child how they get to be big now and go to school. Show them where their things will belong. Find out what they will be doing on their first day. Most schools have a schedule with themes and activities mapped out. You can talk about how they get to paint today, or they are learning about clowns. Get your child excited about what they will be doing.
When the day arrives to take them to school, don't make a big deal about saying goodbye. Help them hang up their things and take them to their teacher. Give them a quick kiss or hug and say goodbye. Then turn around and leave. This is the hardest part - walking away. Dragging out the goodbyes only makes it more emotional for your child. If you don't act like you will be missing them throughout the day, they won't act that way either.