MOROSCO--
The difference between "Chitty..." and shows like "Frog and Toad", "Charlie Brown" (which, I'm guessing you mean "You're a Good Man, Charlie Brown") and "Seussical" is that "Chitty..." is using the old material from the movie with new things thrown in there. People know going in, what they're going to see-- and they know there will be a flying car...
With shows like the three you mentioned-- they were taken from ideas popular with children, but were just used as the foundation to create a musical. The people who saw "Chitty..." (or even ACE VENTURA) remember the title song, and TOOT SWEETS, HUSHABYE MOUNTAIN and such songs like that. "Seussical", "...Charlie Brown", and "F&T" had very little material that people KNEW they were going to see (exceptions of the initial chracters and some of the story).
And if she'll say, "My darling, I'm yours!" I'll throw away my striped tie and my best pressed tweed, all I really need is the girl...