The family friendly section is the Nickey Line from Hemel suburbs 4.5 to Harpenden - though the surface is mainly rough (or muddy, depending), which can be tiring over several miles riding.
At Bulstrode Cross you can ride straight ahead to Felden and Hemel, but L and R on Longcroft Lane is quieter and more interesting. Descend Felden lane carefully to take byway R 2.4 on Upper Roughdown Common. Over M41 and Euston mainline - steeply - (L for Hemel Hempstead) to London Road 3, Grand Union Canal 3.1 and the end of Spoke 10 canal, then L Heath Lane and Cotterells to bypass the town centre. Cross A4145 by pedestrian crossing 3.9, R on Midland Road 4.2, climbing to L the start of the Nickey Line 4.5.
This is the first of the Hertfordshire branch lines. Navigationally straightforward now to Harpenden except through Hemel's industrial zone; watch out for L turn 6 and shortly after the resumption of the offroad track. NCN 57 signage keeps you off-road round Redbourn, where the bypass has captured the railway track bed. The following miles up to and around Harpenden are particularly rewarding. The run ends in a flight of steps 11.7 up to the road bridge over the Midland main line.