Baby Bok Choy is actually a misnomer as it implies that if left alone it will grow up into a regular cabbage. It won't. In Chinese it is 小白菜 (Mand: xiǎo bái cài; Cant: siu2 baak6 coi3) , which is literally 'small bok choy'. It, like the Shanghai bok choy, is cultivar of Brassica rapa subsp. chinensis.
This is one of my favourite brassicas . It can been cooked or eaten raw in salads.
Also, may I mention here that vegetables are almost never steamed in China - they are stir fried, preferably in lard (ie pig fat).