Last Year Jon and I were in Nottingham at the same time as the Robin Hood Festival was going on in Sherwood Forest. It was great to see all the little children dressed as Robin Hood and Maid Marion.
Sherwood Forest is about 40 minute drive from Nottingham and Nottingham is about a 2 hour drive from Cambridge. While this was a great time to see Sherwood Forest, I would suggest not driving all the way from the bases for the day to see the Robin Hood Festival.
The festival going on made the trip to Sherwood Forest worth it because otherwise you just go see the Major Oak. There were all sorts of displays, falconry, jousting, games and other stalls. It was free to attend but you had to pay for parking. You could make this an affordable day by packing your own picnic and then let the children run free.
Also take them walking around Sherwood Forest and see the Major Oak.
Great festival but a bit of a drive for one day.