https://www.nationaltrust.org.uk/borrowdale-and-derwent-water/trails/borger-dalr-geology-walk-borrowdale
This walk isn't too far away from you, bottom end of Derwentwater - I've done it a few times and it seems pretty manageable, you certainly see plenty of families with kids doing it, and the older ones should enjoy the clamber up to the top of Castle Crag, which is a sensible height (nothing overly strenuous) but gives you some great views. Probably not one for the very littlest ones, but someone can always stay with them and paddle around the river while the older ones take on the climb. And there's a couple of decent cafés in Grange too.
Definitely recommend sniffing round the National Trust website for similar walks (including more leisurely ones that everyone can do), there's some good stuff on there. Anything around Derwentwater tends to be pretty lovely, even if you just stick to the lakeside and enjoy the views from ground level.
There's also a surprisingly good tapas and beer place in Keswick (http://esbarkeswick.co.uk/) but probably not for a family trip. One to remember for when you're next back though.