I used free printable clip art for the smiley faces and copy it on yellow construction paper or cardstock. You could also print up the happy faces right from the computer. With the children so fascinated by emojis, it might be fun to print those up and let them choose. On a sentence strip, I wrote the words "I had a happy 1st day in VPK! (The voluntary pre-k program that I teach in my state)
Of course, not every child is happy at school on the first day, but I do have this written for each child. If a child is sad, we talk with them about SOMETHING that was happy about their day. Sometimes it is as simple as the snack that they had, but we can usually find a reason for them to smile.