okay so tuudri, tadhg, ciarán and their (still unnamed) brother are all about the same age. they were born aboard the cú na mara, where they spent most of their early days learning how to pull their weight aboard a ship (obviously not to the point of actual child labour, but they were certainly taught how to do basic tasks like tying ropes or navigation at sea). fiach didn’t want to pen them into healer roles so he held off on advanced healing magic education, though he did teach them basic stuff like how to heal small cuts and which herbs do what, that kind of thing. iora, their mother, taught them the basics of survival - how to hunt, how to fish, how to fly. but even with her skillful education, the children did not set foot on the mainland until they left the ship for the first time to seek out their own clans.
so i guess these kids would probably be kinda... clumsy on land? they have the appropriate knowledge to survive in a forest or desert or whatever, but not the actual life experience. but their nautical experience kind of makes up for it. they might have trouble learning how to navigate air currents over land as opposed to over sea, that kind of thing.
fiach makes absolutely no distinction between biological kids and adopted kids, they’re all just his kids. so tadh & co are actual siblings of zeta, luke, and delta. fiach’s children from before he met luke and delta (dubhcobhlaigh and her sisters) are still thought to be dead.