Children and infants are not allowed in the emergency exit rows of airplanes. However, it only provides a guarantee that a child won’t sit next (laterally) to you — one might still be behind or ahead of you in a non-exit row. If you can find a plane where the exit rows correspond with a change in class or there are multiple exit rows, you might be able to build more of a buffer. 

Myself, I find that earplugs or a good set of noise cancelling headphones along with good music or a movie of my choice more than drowns out the world. If you’re really bothered, in-ear earplugs combined with over the ear noise cancelling headsets (or in-ear noise cancelling headphones combined with over the ear ear protection) will give you around 40dB of noise reduction - more than needed for most children. 

Finally, parents of small children often can’t afford to fly business or first class. There’s also greater seat separation and/or privacy partitioning in those classes. You may find that one way that airlines offer a “choice” (albeit expensive one) to business travelers who absolutely need to get some sleep or work done.