I saw it when I was 4 back in 1995. it was my first Broadway show and i immediately fell in love with it...I don't know exactly why because I'm sure I couldn't understand most of it but I think I fell in love with it mostly because it was new to me. Seeing something live as opposed to a movie or television show is real different. The spectacle and the effects, songs and vocals captivated me. And it was then that I fell in love with theatre.
I would think it depends if the child is old enough up to an extent. for example: a child under 7 or 8 who was exposed to the word "whore" or "b*tch" etc could go into the theatre and well, not understand any of the things said or mentioned. So that's probably ok. They could just enjoy the effects and the whole idea of it being "live." I would mostly think they wouldnt care what the story is about, maybe they'll even think it was "cool" or something because there was the barricades going up. I wouldn't think they would listen a whole lot on the lyrics. maybe to the medely and instruments but most likely not the lyrics.
But if the child is like 9 through 11 or something and they might have to be mature up to an extent because during those ages, i think, they're striving for more understanding and during those ages, they have the ability to pick up the words said/sung.
...well, these are just my opinions.
Back, he spurred like a madman, shrieking a curse to the sky. With the white road smoking behind him and his rapier brandished high! Blood-red were the spurs i' the golden noon; wine-red was his velvet coat, when they shot him down on the highway, down like a dog on the highway, and he lay in his blood on the highway, with the bunch of lace at his throat...
(The Highwayman. Sung by: Loreena McKennitt)