Ok folks, I have a plan of sorts and the ride will be based on the Tissington trail.
For those who don't know it, it's an old railway which is slightly downhill as you head south. No technical skills needed.
Most kids should be happy riding the uphill bits, and I know that the surface is suitable for towing a trailer.
Park up at Parsley Hey (Snack Bar).
Various options from there stamina dependant.
Out and back to Tissington snack bar for little ones(least steep section of the trail 1:263 iirc) 20 miles.
S on High Peak trail past Minniglow then towards Biggin, left to Tissington then back up Tissington Trail. ~26 miles
Have a scan of the map and let me know what you think.