Many think this is an accident waiting to happen... cycling down the only main road in/out of Skagway. Also if it's rainy and cold it won't be too much fun as the road becomes slippery. Tour guides keep the pack together so if you want to race down or your an avid cyclist and want to go your own speed, you won't be able to, you have to keep with the pack.
It's up to you and how much you want to do this excursion, you should always think safety when booking any excursion involving physical activity.
I guess I sound really negative about it but the groups we saw doing this excursion looked totally miserable (was a very cold wet foggy day) of course not all days are going to be like this either! I personally don't think it's safe for under 12's on this tour.
On the other hand there are other bike tours that are much safer. We cycled around the coastal trail in Anchorage and had a great afternoon.
Make your own mind up and have a great time what ever you choose.
We did the Klondike bike tour during our 7/2 cruise. We did have a brief shower but most of the time it was sunny. And the view was beautiful. Did not find that much traffic. We enjoyed it. Just use common sense.