Definitely an interesting number, but I found these other 2 articles from different outlets. These say '16' and '9' respectively. I also noticed that the quotes, from 'witnesses', used by the articles interesting as they appear to be highlighting different words. It's difficult to describe, it's just what jumps out to me. Last of all, it's odd to me that the driver was "arrested on suspicion of causing death by dangerous driving" while also claiming "The driver is believed to have suffered a 'medical incident'".
https://www.dailymail.co.uk/news/article-12270455/Car-crashes-primary-school-Wimbledon-people-hurt.html
https://www.mirror.co.uk/news/uk-news/breaking-wimbledon-incident-land-rover-30403000
"…a woman aged in her 40s…" is all that is stated.