Engagement advice on Creative computing
Show them the cool stuff they can do with Scratch.

Don't focus on explaining that they can't make a AAA game. Focus on showing them examples of stuff that is engaging and interesting, that they can do with Scratch. Come up with a showcase of interesting programs. Make sure your assignments are engaging and interesting.

Taking a step back, I'd also recommend you give your students the benefit of the doubt. Don't come in with an attitude that assumes they only care about modern video games and smartphones. Connect your lessons with stuff they do care about. Have them draw a scene from their favorite show, or have them make a game or animation about their friends.

And taking another step back, the age range of your students is very high. A 6 year old and a 16 year old are very different. I'm not sure Scratch is the right choice for 16 year olds. Maybe try something like Game Maker or Processing? I've mentioned Processing in a bunch of my other answers, and I'd be happy to talk more about it with you if you're interested.