Used mine with my daughter from the ages of about 4-6, for fairly gentle off-road stuff and ferrying around town when the car wasn’t available.
They feel fine in use and have a decent level of off-road capability (although I’d echo the above qualifications regarding length and weight). The addition of a padded seat sounds like a good shout.
Attachment to the seat post didn’t cause me any issues.
The biggest drawback, again noted above, is the lack of stability when stationary. When you stop, you basically have to dismount, about turn and straddle your rear wheel, pinching it between your legs to hold the bike and trailer up as you unbuckle your passenger.