This kid is 6 yrs old! Some of the suggestions are not age appropriate. Has your child been to a live theater show before? Full length? I would strongly suggest you look for mini-shows or something that is geared specifically toward kids (like the Teddybear concerts at Kennedy Center). Something that is 1.5 hrs or less.
The hyenas in Lion King are very scary as is Mustafa.
Maybe your child is unusually mature, OP. I didn't have that luxury... so I keep it simple for my kids. Alladin, Seussical, Annie, Frozen, Mary Poppins.
But, even with these age-appropriate shows, many kids won't have the attention span or interest to sit for the whole thing. If you spend $300 for two tickets... are you going to regret it if your kid is asking you 100 times -- when is it over? I want to go home! ??
This looks like a kid-pleaser and it's only 1.25 hrs.
http://www.newyorkcitytheatre.com/theaters/newworldstages2/the-gazillion-bubble-show.php