In the name of six Immortal Elders they better be a secret. Or a sheetstorm a la Doomsday trailer might happen again.
Let it be a nice surprise.
Kids will appear in trailer and ads, absolutely. We already saw a glimpse of them in Comic Con trailer and I think Mary was at the scene when bullies beat up Freddy.
Plus, a large cast of kids (and diverse) is an attractive point for audience right now, and really sell the 'family movie' aspect of 'Shazam!'. It and Stranger Thing but a superhero movie, that's a unique, distinct selling point right there.