do you mean to Threlkeld and round the blencathra valley head ?
my wife’s been round it and she’s no biker. Main climb to the head of the valley is longish but technically easy even when off-road.
Next bit is a short ride through a bog, followed by climb/walk up a running stream (IME) with wet feet pretty much a safe bet
After that, easy running again apart from one off-camber rocky bit that has a pretty steep drop at the edge – should be walked by your kids but I doubt they’d try it on anyway if they’re cautious types
total length 15-20 I’d guess but fair bit is “flat” railway