There's Alaska Canopy Adventures in both Ketchikan AND Juneau, Alaska Zipline in Juneau, and the newest zipline run by the Icy Strait Point company. You'll probably have to call all of them and just ask directly.
It's got a little to do with legal liability, although mostly it's a weight issue. If your 7 y/o is much less than 50 lbs or so, there might not be enough momentum to get the lines going, and they could get "stuck" in the middle... Also, there's an issue of leg strength to "catch" one's self at the bottom.
If it's truly a "no-go", you'll want to include the young one in the alternative planning... maybe steer their interests a bit toward fishing or whale watching?
The Duck Tour in Ketchikan is just plain "neat"... riding in a bus that drives into the water and becomes a boat. I'm not a fan of bus tours, but my kids and I get excited every time we get to do that one...
Better not for now... signatures get me banned!