If you do go to Penrith watch the Parking, tickets are issued with great ease..
Penrith itself is perhaps not a No1 destination for visitors, but within a few mins drive there are some of top spots..
Take the A66 from Penrith to Keswick (about 20 mins of great views) Head for the Theatre car park, pay and display. Walk down to the lake and feed the ducks, geese, swans etc with the bag of bread you boughht with you (DS will love this..!)
Now take the short walk to Friars cragg, splendid views. If its a nice day hire a rowing boat for a row round the Island, also a small cruiser type boat goes round the lake.
For lunch walk into Keswick (thro underpass), the first cafe on the right is called Masons, do very good wholefood, another 20 yards on the right is George Fishers, cafe on top floor is excellent with kids.. Our children love a Keswick day.
The Ullswater steamer may be of interest, park at Glenridding.
A walk up to the waterfalls at Aira force is worthwhile.
Eden Ostrich world may be a little old for DS.
Avoid the softplay at Rheged in August, complete nightmare in August..
Check to see if there are any village shows on, these are always brillant and worth seeking out. If you post your dates I will have a look..