There is lots to see on Hollywood Blvd., such as the Roosevelt Hotel (down the street from the El Capitan) where the first Academy Awards were held (1929). Also Ripley's Believe it or Not, Madame Tussard's, a Guinness World Record museum . . . those might be appealing for your boys? I find the "characters" parading in front of Grauman's Chinese Theater too aggressive, but you can easily get around them and tour the hand/food prints across the street by the theater. There are some nice places to eat in the Hollywood & Highland Center, too. Plus, there are some fun fountains and other things to look at there.
Also, on Hollywood Blvd., there are some truly classic "old Hollywood" restaurants, like the Pig 'n Whistle (dates back to the 1930s), and Musso & Frank's Grill (dates back to 1919). I'm not sure how close those are to the El Cap theater, though. You might have to drive and park again. However, if you're there for the whole day, it might be fun to see that end of Hollywood Blvd. There's also the Pantages Theater, another site for the Academy Awards (1949-1959) and a great Art Deco masterpiece, too.