How long should a children's book be? Where should I get mine published?

I'm a cartoonist and while I do a lot of digital artwork, I'm not familiar with how publishing a children's book works. The original story consist of colorful and odd characters going on this adventure and meeting many characters along the way, I just don't want it to make it too long. My artwork I always tend to use bright colors and focus on making my character designs easily recognizable and fun to look at. How long do you think a children's adventure story should be? Also, since this is my first time actually writing a book, I'm curious on what I can do to get it published and out there, but protecting my idea at the same time. I'm sure there are much more experienced writers than me on here, so I seek advice from the more professional. I really appreciate it!