There are wings and children't kits inside the blue Customer Care Kit on every flight. Ask the flight attendant to give you some. The only rarely (and with a good and unusual reason) seen a pilot deny a child a flight deck visit. These days, pilots usually show up midway through boarding and leave with the first of the customers so you may have trouble getting to the cockpit when they are available. If you board around group 3 or 4, they'll usually have arrived. Have your cell phone ready to take some photos.