In reply to Huds88:
That’s a broad area you give; you don’t say where you’re based. I’d start local so you don’t heap expectations on the trip on and the little one. Engage them in building up to a big climb late in the autumn?
Attitude to the walk is far more important than the location IMO. Look for somewhere they can use a loo on arrival, make sure shoes fit well, pack lots of snacks, have lots of little snack breaks, beware that if their enthusiasm is lost for any reason each new footstep up will be literally 10x harder than their previous but turn around and they’ll be running… It helps if there’s an icecream shop at the end. Also be aware if you don’t know how they’ll handle exposure, particularly if it’s windy (as it inevitably is of late!).
I’ll second (or third) Rosebery Topping; toilet block and an icecream van in summer. Levers water from Coniston is a good outing that immerses them in mountains, you can then make a call based on pace, attitude and weather on if you continue up or descend, with loops available for both. The Howgills are very easy hills but the loops are quite long.
As long as you feed and water them, active kids are almost never limited by physical endurance but the mental endurance which you have to play to - and recognise when it’s gone, which is why I’d avoid long drives for the first attempt so you don’t find yourself pushing to hard against their tendencies.