If you want them to know both (modern Greek and reconstructed) you may want to start them on the reconstructed sooner rather than later. It may confuse them for a bit but the younger the kid the easier for them to handle it.
See, I'm afraid that if you wait too long they may decide they don't want to use reconstructed at all. Much like me. I know it, I can and do use it if needed, but I most certainly don't use it in general. Mind you my native language is Greek and I was taught ancient Greek using MG
Of course the problem is that I don't think there are many resources for teaching ancient Greek to young children.
Not much help I know, but maybe some of the other members will have better advice to give you