Here's
NYC Century suggested training.
Basic idea is to build both base miles as well as a single longer ride. I've used this to train for a metric century and it worked well. It sounds like you've got your base miles down, but you will want to have some longer training rides in there.
The only comment is make sure your actual ride isn't on more difficult (
e.g., hilly) terrain than your training.
Have a good time. It's a nice thing to do with your kid.
