And to prove to myself that I could, I did it. I went the "full Meredith Willson" and wrote the book, music, and lyrics myself. I wound up with a book that more or less made sense, and 23 songs, some of which would surely have been cut in production. But I never sought to have it produced, partly because I'd only written it to prove to myself that I could do it, partly because I knew it had no commercial potential, and partly because I knew the book was weak and I didn't feel like re-writing it.
From my experience, I can tell you that--relatively speaking--writing music is a joy, writing lyrics is harder and trickier, but great fun, like solving a complicated crossword puzzle in four or five dimensions (rhyme, sense, meter, character, emotional accuracy in the moment, etc.) at once, and writing the book is hell!!! I learned to have enormous admiration for people who can write decent Broadway musical books--of whom I am not one.
I never consider writing musicals for children, because I consider the best existing musicals aimed at adults to be the best introductions to musical theater for children. Brigadoon, Song of Norway, Kiss Me Kate, South Pacific, Oklahoma!, and The King and I, which I was taken to see in various California revivals between the ages of 6 and 12, were my own introductions to musical theater. I'd recommend a similar program to anyone wishing to introduce a child to musical theater today.