Salads are hard for little ones to masticate. My 4 year old will now eat salads, but it took a few years for her to be able to eat it properly without choking a little.
One thing we did was get a juicer and juice the collard greens and ohter green leafy things. I know it sounds yucky, well it does to me, but our daughter would suck down those juices like there was no tomorrow. She likes kale, collard, apple, carrot juice a lot.