The first day goes best when it is not a surprise. A few small habits in the weeks before make the morning calmer for everyone.
Practise the journey
Walk or drive the route together, and say what will happen at the gate. Children cope better with what they can picture.
Name the small jobs
Putting on shoes, opening a lunch box, asking for the toilet. Practise these at home, and celebrate them.
Keep goodbyes short
A cheerful, clear goodbye works better than a long one. Teachers are used to tears, and they usually stop within minutes.