We (me and 4 yr old ds) did this yesterday. The theme this month was hot and cold. There were lots of experiments that you could do, including making ice cream and an ice cube necklace, plus interactive displays. They also had lectures throughout the day. The 'cold' lecture was not that interesting for a 4 yr old (lots of pouring of liquid nitrogen to freeze things) but the pyromania talk was superb and included setting fire to a digestive biscuit (which got so hot it burnt through steel gauze!) and exploding jelly babies. As well as showing how the colours are put into fireworks complete with the end result!
They have family fun days every first Saturday of the month. This was our first but I think it wil become a regular excursion. Entrance is cheap too - £5 for adults and £3 for children and they have a nice cafe and restaurant on site.