Best greens for calcium are kale (and all mine also love it) and collard greens (which all of mine hate lol). You can also try broccoli, turnip greens, garlic cloves (which mine will fight over cloves of garlic and it's good for them in other ways), or mustard greens (which mine will eat... but only grudgingly) if your birds are picky or get bored easily. Mine also like a small helping of plain greek yogurt once in a while.
And the reason Zaz says 'not absorbing' (because you may have oyster shell or something in the pen, so she may be eating enough calcium) is because calcium requires other things in order for a body to absorb it (which is why it's so much less effective to give straight up vitamins without the enzymes and proteins and stuff found in natural foods), so eating it WITH those things can help with the uptake. If you're giving them treats or they have access to free range Stuff (I can't remember if your kids are penned or not sorry!), then something else she may have eaten on a particular day may have helped in temporarily inhibit her ability to uptak calcium, if it's just the one egg.
Hmm, well my birds are penned but their runs are 20 x 30, for 3 birds. The vegetation is quite thick and there is dandelion by the ton in there, I see them eat that along with clover and plain grass. My kids are boogers when it comes to greens, they will only eat it if they picked it themselves. Even in the middle of winter they will turn their noses up at any cut greens, kale, dandelion, cabbage, spring mix, spinach, all have been tried and left to wilt on the pen floor unfortunately. That is the main reason I got the alfalfa dried flakes, to mix in, thought I could sneak it by them. It sounds like egg shell calcium would be the ideal, but mine were not raised with scrambled eggs as a treat, and usually they don't eat them. Is there any sort of powdered egg shell supplement that could be sprinkled on potato bread, as this is the one thing they will kill each other for?