West Marine sells them, but they are spendy:
I've been tracking Ebay to see if they come up for less $$$. Surf's idea isn't bad, but the life jackets my kids wear (Mustang) aren't set up to withstand a shockload. The West Marine harnesses look safer and look easy to wear under a life jacket.
Of course, I'm thinking of going offshore with kids. For the protected waters we sail, we don't worry about harnesses for the kids (age 7 and 9)-- but they always wear lifejackets.