Is there a way to click a button and send all of my career horses to work?? Or can we add that to the game eventually? Also, how do you mass train horses? 

You have to add a tag for them, 

Estate -> Manage -> Tags ->

To put them in a show:

Go to the show you want them all in, and click filter horses, click enter horse, and then those horses with the tag come up.

To put the tag on the horse:

Horse you want to put the tag on -> Horse Editor -> under the horses name -> your tags are there!

(Oops, put this in the wrong place!)

@katielee did you accidentally reply to the wrong thing? Careers aren't shows.

To career all horses at once you need to own the "enter all 2" item, which can be purchased from the ingot store under "account". I think you need to own enter all 1 first, I can't remember exactly how many ingots they cost. 

Sorry, I did.

To mass Train all horses you also need to purchase the Training Manual upgrade with ingots. Training Manual 1 costs 1,250 ingots. I do not remember how much Enter All 1 costs either.