Whinlatter is great for mixed ability groups imo. 3 good trails of varying difficulty all going out from the same place so dead easy to combine options etc. Blue run is great fun, south loop is not much harder (especially the bottom half – you can miss out the top section) but still great, north loop is a bit harder but not quite as good imo. Fantastic views on lots of it. There's a superb singletrack descent down to Thornthwaite on a bridleway that starts just down from the visitor centre too.
Grizedale would be a good option too but not the North Face Trail really – pick pretty much anyway bridleway in the forest and you will find some easy but fast and rocky fun.
There's a route in the VG guides book round Iron Keld and Loughrigg Terrace that has some great riding and views, plus plenty of options for cutting it short if necessary.
