I was a preschool teacher before having Daniel, so I'm used to being silly in public.
Just BE a kid with them. Think like they do. Look at the bugs they find, build with blocks and get down on their level with them. Make eye contact with them, follow their lead, let them make up the games and rules. Try not to take over for them (dh does this and it makes Daniel crazy).
There's nothing magical to it

just be a KID
HTH