do you recommend bringing some cash in CNYs before we arrive or are ATMs readily available?

It is quite easy to exchange money in China and the rates will be more favorable, but it is advisable to exchange a small amount in your country before leaving.

When you are in China, you could exchange money just at your hotel in Beijing. And you could withdraw money with your credit card from ATM machines, as long as the symbols and logos of your credit card are shown on the ATMs.