Kettlewell is a beautiful little picnic spot just off the Borrowdale Road, on the south east shores of Derwentwater. Free for National Trust members, and offering a pretty little beach and local walks.
Coniston Water is one of our favourite lakes in the Lake District, and is accessible from many areas. Here is the perfect family picnic, based near the Bluebird Cafe in Coniston itself.
Brotherswater, in Patterdale is a beautiful and quiet place to visit. You can do the short, 2.3 mile loop around the water, or if you have very young children, just the walk from the car park to the little beach for a picnic and paddle!