I think you’ll have a great time. Any reason you want to do the HPT rather than the Tissington trail?
For a 7 year old I’d do the Tissington trail, partly because there are some good places to stop – like the sweet shop in Tissington, and it’s a lot less exposed.
As above, the rise up to Middleton Top will be a drag for him, and there’s another though small rise just shortly after. From there it’s a nice but more exposed ride up to Parsley Hay.
If you want to do the 54A route you should be fine as there’s little traffic in Hopton and Bradbourne. Just be careful crossing the roads around Carsington. The slog is the hill from Tissington splash (Ford) up to Tissington…I doubt he’ll like that! Also by linking up on the 54 you loose ALL the height you worked to gain, so I wouldn’t recommend it.
The Royal Oak is a brilliant pub, and the food is fantastic. At 7 he’d love the play area at the Bull in Monyash just down the road (but it’s not great ride back to the Royal Oak from there for a 7 year old. The road can get busy and it’s a very long drag).
Weather is OK here. No rain all day, about 15c and it’s apparently much nicer tomorrow.