N.P.
It's preschool - do nothing. Not one thing except maybe say "we are so proud of the friends you made this year"
That is it. Nothing else.
It's only preschool. It's not like kids fail preschool. It's not a big developmental milestone. Seriously. It's preschool. Get perspective and don't reward kids for nothing.
Oh and just so you know, I RUN and TEACH at a preschool.