Can we buy small cans of prune juice in China? Soda pop? Diet soda?

You can certainly buy cans of soda and probably diet soda. And juices. And at each of the hotels in stayed at, prunes were part of the lavish breakfast buffets. I kept containers of peach juice in my mini-bar to mix with my evening duty-free vodka!! There are big supermarkets in Beijing and Shanghai that stock things like imported orange juice, along with local juices.

They are regurally avaliable as with all developed countries, ekscrunchy is right though, prune juice you will probally only find in hotel breakfast buffets (so cans, no). Strangly, all drinks you find in cans are smaller than in the US.
Note: don't by Lipton Ice Tea in China, it tastes of methenol

There are also plenty of smaller convenience stores that sell juice - though I don't know about (ugh!) prune juice.

You can definitely get diet soda in many locations in Beijing and Shanghai. To be honest, I really have no idea about prune juice! You MIGHT be able to pick some up in the expat stores (like at the Lufthansa center in Beijing and CityMarket in Shangahi).