When you start out by saying you have “reservations” and describing them as “misguided” it does seem like you’ve already decided they’ve made the wrong choice in regard to their children’s diets.
If you go in with that mindset, any conversation is likely to be fraught. At the moment you’re only in receipt of partial information. You need to have a much clearer picture.
I don’t think it’s wrong to feel concerned by the things your granddaughter mentioned, but I don’t think you will get far if you go in with the mindset that “they’re wrong and I need to get them to see things my way.”
If you try to have an open conversation based on the assumption that they are not “misguided” but simply taking a different approach to yours and you’re figuring out how that works, you may be able to have a productive conversation.
So you could bring it up from the perspective that your GD mentioned low iron and dark circles and doctors and so on and you weren’t sure what she was saying, and just ask them. They may have a clear explanation that acknowledges a problem and explains the solution they’re working on. If not, you could just ask more questions in an interested way, eg “Oh, is it hard to get enough iron into them? What are the best vegan sources of iron? Do the girls like spinach?” Etc. I would just have a friendly conversation where at no point do you tell them what you think they should be doing. Afterwards, if you still feel concerned you could send them a friendly emailing saying you found the conversation so interesting that you did some further reading and have they seen these “iron fish” items etc.
If you are going to make suggestions, look for ones that support their choice. Not “oh look, I found this link that says growing children need to eat meat to get enough iron.”