Please indicate age and terrain.
I'll start with Pen-y-ghent - its not too long a trek to get to the start of the climb (if you park on the road in the right place
) and the climb is very scrambly. Child gets good sense of achievement without too much boring slog.
Age - DD was 6 when she first did it, I've seen younger. Does require the child to have enough nous and coordination not to fall off the path in places!