So I did a reconnaissance mission for a potential trip with the chieflets from Balerno to Nine Mile Burn (mentioned above).
The Red Road up from Bavelaw was okay. A bit exposed but nothing a bag of jelly babies shouldn't conquer. Took me 40mins from the reservoir without stopping or walking. I reckon I will need to ration the jelly babies over a 90min time horizon.
At the saddle I took the lower route to Nine Mile Burn rather (via Braid Law) to see if I could avoid the extra elevation gain from the Monk's Rig route.
That was mistake, as was continuing across the track heading to the farm and thereby going the wrong side of Braid Law.
The route itself wasn't too bad on the way down but was technical enough (tree roots, muddy divots etc) that the return journey would be tough work on the arms as well as the legs.
On the wrong side of Braid Law it was still a path but getting narrower between bracken. The worse part was the cross slope, which was enough to catch the uphill pedal at the bottom of the stroke. Not sure it would pass the risk assessment for little people.
I had to back track a bit to get down to the wider path heading to Quarrel Burn Reservoir and then a push uphill to regain the proper path.
The last bit was some nice grassy slopes to Nine Mile Burn. Feeling peckish, I then traversed to Carlops in search of food. Another mistake. The pub was shut :-(
Back up the hill and I followed the Monk's Rig route up. Mainly grassy slopes with a few parts that were beyond my legs and I was pushing. May need an extra bag of Jelly Babies but still only took me 40mins.
The descent back to Thriepmuir was lovely, I even took the rooty route through the avenue of trees. Proper Mountain biking. :-)
In summary, I reckon it would be doable with the chieflets, with enough bribes. The ride up to Balerno could be a grind before we even get to the hills but the northbound journey should have a great effort / reward ratio with it being downhill from Balerno home.
Now we just need to work out a date and pray to the weather fairy.