The NFT is not representative of Grizedale! There is loads of fabulous riding if you bother to read a map. Also links very well with other surrounding routes such as Claife, Tilberthwaite / Hodge Close, Iron Keld etc.
My 11 yr old likes Grizedale a lot - a usual ride with him would be start at visitor centre and use first 2 sections of NFT to gain some height. Then turn off onto Bridleway to Parkamoor (great open views of Coniston). Then Bridleways towards Satterthwaite - excellent but quite rocky and techy descents (need to go carefully but they aren't impossible - my 9 yr old has rolled them on rigid & 24" wheels provided I slowed him down before the rough bits). They get progressively rockier / harder so just gauge his ability. If he's tired then don't go right down to Satterthwaite - just contour using forest roads back to VC.
If has energy, then Moor Lane down to Satterthwaite, up Breasty Haw. Do first couple of BW sections but don't drop right down to road on other side - unless he wants another climb :-). Various options of forest road / short section of bridleway to "The Fox" then he'll probably be ready to head back to VC.
Whinlatter isn't worth the trek from Eskdale.
Eskdale and Duddon valley are nice as already mentioned - some of the nicest and quietest bits in the whole of lakes. Don't bother with Harter Fell. Some good stuff heading over to Broughton in Furness.
If you have a bit of spare time then for something different can also go up Ennerdale but would just be a fire road out and back. Making it a loop becomes an epic (Scarth Gap / Black Sail pass).