First, have their fecals been checked for parasites, coccidia, and unusual bacteria?
I also would be concerned about them not getting enough nutrients with just hay, but I think Randy has some on a diet like that. Are you giving them different types of hay? That helps a lot.
The first veggie I start any rabbit with is parsley. It has a mild calming effect on the stomach, is very tasty, and has lots of vitamins. Start with just a tiny bit. Another good starter is mint or any mint-family herb like basil. These are all very good for upset stomachs and also have good nutritional value. I drink peppermint tea to help with my IBS and it does help. These are also really easy plants to grow in pots. Just don't plant them in the garden because many mint species are invasive and will take over the whole yard!
Hope that helps! Do be aware that some rabbits just do not handle veggies very well. How are they with pellets? Even just a tiny smidge a day can provide some of the vitamins and minerals they might be missing.
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