rental boats on canals in France

What time of year are you going? That will make all the difference. Any of the canals in France are going to be quite crowded during the summer months...there can be quite a bit of jostling and waiting at locks and docks. None of them are terribly difficult to maneuver that I have seen.

We are going the second week in September.

You're past the peak crowds which are only bad on the Midi, anyway.

You get a short course on how the boats work and a bit of supervised driving practice.

My kids were rerunning the boats alone when they were eight or ten.

Thank you for your information. I feel more confident now.
We will go for a week from Hochfelden to Gondrexange and return. Do you know about the market days on the towns we will pass?

These links can help you plan market days:

http://www.tourisme-alsace.com/en/the-markets/

http://www.rhine-online.com/html/france/shopping-alsace/market-days.htm