Ok, I need a little insight from the veterans.

I have never done the ride before.
My wife and I want to take the kids on the two-day. I know that our 9
year old could complete the 20-30 miles for each day. My issue is our
7 year old. His bike is just a 18", single gear. I don't think it is
fair to try to make him ride that distance on that bike. He isn't big
enough to ride a multi-gear bike yet.

Is there an option to have him start 1/2 way or something like that?
Any suggestions? Should I just wait a year or two?

If he were little, I would just put him in the bike trailer and pull
him along.

Open to anything.....thanks.