The only way you can get kids to watch older films...
...is if you, the parents/guardians introduce such movies to them and show them how to appreciate the stories, even if they're dated.
I would have never known about all the sci-fi classics if it hadn't been for my dad introducing them to my brother and I when we were kids, for example. Or we would have never known about classic, live-action Disney films if it hadn't been for my mom and grandparents having us watch them as kids. Heck, I wouldn't have even known about Shirley Temple if my grandma hadn't been a fan.
I've stated before to the younger generations, that just because it's an older film, doesn't mean the story isn't good. Sometimes you can appreciate a good story, regardless of the age of the movie. Plus, it gives you some insight on the culture of the time period it was made.