Yea – every instructor I’ve spoken to has said wait till they are 8/9 due to them being too fragile.
From what i see my 5y doing at the park, I would think she could take it but I’m completely guessing.
End of the day I recon sking is not a bad thing and it will probably make them a better snowboarder when they get to 8/9y.
If you do, make sure you base it on the snow conditions. Even the most skilled sports person will fall hard a few times when learning which is not a big issue when the piste is soft but if it’s in anyway hard – stick to sking and wait till next trip.
Last year a good skier in my chalet asked me if i could show him the SB basics for a few hours. I said, normally yes, but not tomorrow as it was due to freeze over that night and the piste/beginners area would be bullet proof. And because he was a victim of fashion, he also refused to wear helmet or kneepads (which in my opinion are essential for the first few days). Anyhow he got a 4 hour private the next day and broke himself after 1 hour – saw stars – bump to his head, black knees, couldn’t sit down – really screwed up the rest of his trip.