I started taking my son when he was 2 (He's 5 now
).
He loves trains, so we always ride Marta down from Lindbergh. Which is easy parking on Saturdays. If you get off at North Ave and go out the north exit to 3rd street, it's a block to the tunnel under 75/85 and then you're pretty much right at Callaway Plaza. We always go early to see the band and the pageantry. He LOVES watching the Wreck drive out and the cheerleaders and everything. Could not care less about the game though - why are bunch of grownups running around and hitting each other? So, we've never made it to halftime. We go see the fun stuff, watch as much of the game as he has patience and I catch the rest of the game on the DVR when we get home.
We've been able to 'sneak in' some obvious kid's snacks without much trouble in the past. But, like a baggy of cheerios not a big picnic. But we didn't go at all last year, so they might be more strict.
We have season tix, so I don't know about whether he needs a ticket. If you're buying tickets, I recommend sections 222-224 northeast visitor side upper deck, in a low row. Has the best view of the Wreck coming out and it's the easiest in/out from Callaway Plaza. It's rarely full or crowded so you can spread out a little.
Have fun!