We are travelling from South Africa to Denver to visit my son but we have a stop over at heathrow for 11 hours, my granddaughter lives in the uk, could we pass through security to see her? We are traveling from the same airport and understand we will need to go through Security again, we have an American tourist visa, would we need to get another type of visa even if we don't leave the Airport?
It appears you are of South African nationality, have a valid US visa and will spend less than 24 hours in the UK. This means you are probably eligible for the transit without visa concession. You must apply for this concession on arrival, and while it would probably be granted, it may be denied, in which case you'll either have to remain airside or (if something else is horribly wrong) be detained.
To be sure, you can apply for the Visitor in Transit visa before your trip.
The UK has two kinds of Transit visa. The Direct Airside Transit Visa only allows you to remain 'airside', i.e. it does not allow you to cross immigration. The Visitor in Transit Visa allows you to cross immigration, and allows you 48 hours between arrival and departure. You can sightsee or visit family during those 48 hours. Some nationalities do not need visas at all to transit.