I got one from Rutland Cycling via ebay.
My daughters 8 and uses 20″ wheels.
It’s fine on most bridleway type stuff – secure and she can pedal to help. Not as good as the trailer bike we also have but it does mean she can propel herself on the flat bits and also ride roudn campsites etc when we’re camping.
I’ve had a probelm when the terrain gets a bit ‘tech’ in that her front wheel/the trailer bar can catch my back one going over the lips of drops etc (this is less of a problem with the trailer bike as she’s further back and there’s a bit more clearance.
On balance I’d recommend for family trips if;
a) you don’t have/want to take a trailer bike
b) you’re not planning anythign to adventurous.