Kids' Lunches
Lunch products for kids, such as Lunchables brand lunches and Kid Cuisine brand meals, often have instant-win games aimed at getting the attention of kids. Product giveaways, which usually come with a grand prize such as money, a scholarship or some variety of trip, are also common. These aren't always going on, but instant-win games that use a kids' food medium are pretty common in the grocery store.
Collectible Card Games
Though there are adults who play collectible card games like Yu-gi-oh and Pokemon, along with Magic the Gathering, the contests that are offered occasionally through these games are often aimed at a younger audience. Free cards, T-shirts and usually some variety of grand prize are common in these games as well. All you need to do is purchase a pack of cards while the contest is running and see if you have a winning entry.
Soda Games
Coke and Pepsi, along with other brands of drinks, often have instant-win games going on. Some of the prizes, such as car giveaways or other prizes that you need to be 18 to 21 to even use, are exclusively for adults. However, kids who win instant prizes like a free Coke can usually claim the prize they find under the cap.
Open Games
Lots of instant-win games are open to all ages, and not exclusively to kids. If the rules have a caveat that excludes kids, such as you must be 18 or older (or 21 or older) to play, then obviously kids are excluded. However, if you find an instant-win game without that stipulation in the rules, then kids can play, and win.