Once Upon a Time, a Chipmunk and a Penguin Went to a Motel Room

I'm finishing an illustration-only book. It was intended for children, but it's suitable for all ages. 

Please refer to this post.

So would it be considered a children's picture book because it meets the page-count criteria, or could it be stretched to the novelty category and submitted to agents that don't accept children's fiction? It seems to me that novelty can be a tough sell, but aren't consumers more likely to purchase a novelty/gift book than, say, a fifteen-dollar picture book? I ask that realizing your answer most likely is that it depends on the pictures, but feel free to surprise me here.

It depends on the pictures---and the topic. 

There is a core audience for your book.  I'm guessing, from your question, that the topic or treatment is somewhat adult, and the only reason you think it might be a children's book is the format.  I don't suppose you've seen Baby, Mix Me a Drink?  Or Furverts?  Those are both board book formats, a format associated with infants and toddlers.  Does the format make them for that audience?  OH HELL NO.

Of course, there are some picture books published every year by children's imprints for which the audience is really adults.  The ones who skate that line in an acceptable way are usually light-hearted life advice, like: "if you love someone, set them free."  They are bought as graduation gifts (see Walk On or Oh The Places You'll Go).  The ones that don't are usually dreadful and sometimes psychotic life advice, like: "if you love someone, let them chop you down to a stump."

But graduation gifts is a difficult niche to publish into---more difficult than adult novelty books. 

Figure out who your audience is.  Good luck!