Publishers and Imprints?

Depends. Take Random House in the UK: it has a whole bunch of imprints like Corgi and Red Fox which all come under the Random House Children's Books label and are all run out of the same offices by the same people. There would be no point submitting a book to both Corgi and Red Fox. On the other hand, RHCB also contains Andersen Press and David Fickling Books, which are run out of different offices and have more autonomy - something sent to Andersen wouldn't be submitted to Fickling or Corgi, for example.

Imprints can, therefore, be rather a tangled web, and I don't know exactly whether Clarion, Harcourt and Houghton Mifflin have anything in common besides ownership. It looks, however, as if Clarion are in New York and Houghton Mifflin are in Boston, so in this case I'd send it to each division (should, as you say, it fit.)

I was recently at a conference with a Dial editor in attendance. Dial is one of the Penguin children's imprints. She said it's okay to submit to each individual imprint where the book would fit, but don't do so simultaneously. It's a sticky situation if more than one of the imprints wants the book.

Anyway, you're not asking about the Penguin imprints, but I think the same general advice would apply.

Good luck!

Smish