A little bit once in a while as part of a varied diet is fine. The diet should be mainly weeds from a safe, chemical free area. We have a whole section for weed ID. Also try hibiscus and rose leaves and flowers, mulberry tree and grape vine leaves, spineless opuntia cactus pads... A variety of leafy greens from the store are okay too. Look for endive, escarole, collards, mustard, carrot tops, cilantro, bok choy, chard, all the different lettuces, water cress, etc...
I've actually heard not to go for bok choy. But as a snack it's ok. They love it. A bit of spinach is in the same category. Not a big deal, just don't do it every day or they might get sick.