I recently took a trip to Stonehenge from London with my son (aged 7) and realised there were a few different options so wanted to share them here to help others and hope that people can also add to them in the comments for a central place.
By Train
Turns out this was more complicated and expensive than I thought originally, you need to take a train from London Waterloo to Salisbury - this was coming out at about £70 total return. Then to get from Salisbury to Stonehenge you can take a bus or taxi, bus was approx. £35 return again for us both so already just on public transport it was £105 - I didn't bother looking at taxis. This doesn't include the cost of entrance tickets which after searching I found some discount tickets for approximately £40. Ultimately total round trip including entrance fees of £145 - I guess not too bad, but at the mercy of public transport and timings which I didn't fancy with my 7 year old son.
By Car
This probably would have been my preferred option if I had a car, but as my wife was using it it meant that this wasn't actually an option for me. I guess if I could then given its about 160 miles round trip I guessed in petrol it may have been about £30 and a hell of a lot more convenient.
Group Tour
I did actually consider this, even as a London resident I thought I could get to Victoria Coach Station where most tour companies seem to go from and take a coach there and back and get entrances included. I found a company offering it for £134 for myself and my son and including entrance fees. This didn't feel too bad especially as the coach has wifi on board which would be great for me if I was struggling to keep my son entertained.
Sites I used to help
If you have any questions ask in the comments, also please do let me (and readers) know if you have any other ways of seeing Stonehenge from London that I didn't consider.