Regarding this incident, I do have a few things -
1. Why couldnt the parent attempt to try other places to try and board the train, IE the front or rear as more often than not, there is a bit more space as commuters often go for the middle coaches, possibly the front too especially if into Waterloo - maybe the rear might have been a better bet.
2. Why, after a few failed chances of getting onto the train, couldnt she have sought a member of staff to get some assistance ? They would be able to help force people to move down inside trains, which is a big issue on busy services as people always clog up by the doors.
3. Further to the above, if you really felt like you were unable to get on, why not attempt to get a bus, or speak to staff regarding alternative transport/methods of travel. Clapham has 2 parts of the station so she could have attempted to use the other side of the station for a service.
4. Why was the OP watching this woman for 15 trains ? They are at about a fequency of every 5 minutes or so during the AM peak from Clapham to Waterloo, so that is sitting there watching for over an hour, do you not have a train to catch ? Why would you do that and also, if you are seeing that they have an issue - why not offer to help ? Also, "distraught", really ? Frustrated, yes. Distraught is a word used for when, as an example, you lose a loved one - not the same as having issues getting on a train.