1. Having run a Montessori School for 7 years, I have experienced many 'first days' and I can assure you that it's far worse for mums than for the children. Whatever they do and say to you as you say goodbye, within five minutes they are happily playing with the other children, exploring the new toys and having a great time. It's often only as the day draws to an end and they think about going home that they remember that they miss you. When the teacher says they've had a great day, believe them. Put on your best happy face yourself and find out about all the fun things they've done.
2. Some children find it hard to settle in because they think it's all so different but in their short lives they have experienced many changes or new things and may need a reminder about how well they have coped with say a new sibling, a new pair of shoes, a new friend, a new activity. They will have a strategy such as perhaps, taking it slowly and carefully or expecting it to be fun. Remind your child how much they enjoyed something new and how good they are with new experiences.
3. You are an important factor. Instead of fearing the worst and worrying about it, which they will pick up on, think about what you DO want. Imagine them happily going in, running in to join friends and having a great day. When you visualise a good result it shows in how you are with your child, they sense your positivity and whilst they may not go in enthusiastically, they will have absorbed some of your confidence about what the day will bring. This is called 'towards thinking' and is about what you want and focusing on that, rather than what you don't want. What you focus on is what you get more of.
4. Teach your child how to anchor a good experience. Ask your child to tell you about some things they've enjoyed, fun activities, toys, books, new friends. Can they think of one nice thing for each day? As they say that thing, ask them to do a 'thumbs up' sign. How many nice things can they think of? Then before they go into school in the morning get them to do this 'thumbs up' sign to remind themselves of the fun things from yesterday.
5. Your child will be either noticing new things (visual) hearing about new things or learning songs, rhymes (auditory) or doing new activities and games (kinaesthetic). Where is their focus? Focus on the visual language with a child who is visual by asking 'I wonder what you'll be looking at today' ask an auditory child 'I wonder what songs you'll be singing today' and for a kinaesthetic child 'I'm wondering what games you'll play today'. The phrase 'I'm wondering' shows you are curious and interested and tends to elicit more from the child than you'll get from a straight question.
I hope this helps. You can find out much more about NLP and parenting in my book 'Be a happier parent with NLP' and there's a useful workbook for your Primary school age child called 'NLP for Children' and for your older child 'NLP for Tweens'.