I stopped at the Charles Darwin Down House with my family of five (children 8, 7, and 1 years old). The route to get to the house is a bit tight with many single lane roads to negotiate without laybys to let others pass. Parking is quite adequate. An elevator and ramps are provided for wheelchair accessibility.
The house itself takes you through a good overview of Darwin's life from childhood to death, with a greater focus on his time upon the HMS Beagle and work upon 'On the Origin of Species" in Down. If you fail to take the audio guide for the ground floor rooms you'd think the display is quite pitiful, so don't skip it. It only costs you about 30 minutes time. We unfortunately skipped the gardens due to rain. Overall, I enjoyed the visit. There were not many connections to be made by my 7 and 8 year old with their studies, but there are many interactive exhibits and they seemed to enjoy the audio guide. Since it was a mid-term break, the Down House was running a Time Travelers activity which they seemed to enjoy.
Worth noting: Staff friendliness is hit or miss, especially compared to other EH sites in Kent and England. One attempted to charge me the £1 cover for the Time Traveller children's activity for my 1 year old but quickly corrected my total when I protested. Staff manning the Time Traveller activity seemed more interested in following strict rules process and procedure than allowing children to have fun. One child was not allowed into the activity and asked to return later. My children were only able to do one out of three activities in the time allotted, but were not allowed to take the activity sheets for the other activities out of the room. Tea room staff refused to serve guests seated outdoors and seemed more interested in chatting each other up and closing down for the day. The tea room also seems to run out of some items later in the day. While any staff unpleasantness didn't spoil the visit, it just seemed odd and unfortunately stood out like a sore thumb.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.