We went today and had a great time.
Cheap Free entry, but parking is £3.50 for 4 hrs or £4.50 for the day.
I was surprised how much there was to do inside: visitors' centre, interactive exhibition, AV film to watch, loads. I'm not sure how crowded it gets in the summer on a wet day, but probably worth bearing in mind as a wet weather option if you're in the Lakes on holiday.
Loads of trails and quizzes and stuff to do if its nice.
Adventure playground was fab but muddy, so take wellies or boots.
The food in the cafe was wonderful, great quality home-made cooking: delicious! Kids' menu is half portions of 'real' food It's not cheap food, but good value for what you get.