At age 5, my oldest son rode 20 kms with me on an old railgrade. At the end, he wanted more. At age 8, my younger son lost interest somewhere between 15 and 20 kms. He'd rather get off his bike and play.

So, it all depends on the personality and fitness level of the child.

When they were 8 and 10, I rented a kiddie trailer and loaded it for an unsupported 4 day family tour. My wife and boys didn't carry anything. Me dragging close to 100 lbs in the trailer slowed me down to a speed that we all could ride happily together.