Children can't judge accurately speed of vehicles travelling faster than 20 mph, say boffins at the Royal Holloway University of London. This won't come as a surprise to you if you have kids. They're pretty clueless about crossing roads. They wait for the road to become completely deserted before they cross, or if not, it's more or less down to blind luck if they choose a good moment to cross.
Which is on reason why 20MPH limits are important, and why it's even more important that they're enforced: the drivers tooling along at 30MPH+ are the ones whose approach rate the kids can't judge, and the ones that will do the most damage in the event of a collision.
The police complain they "don't have the manpower to enforce" 20MPH limits. Of course not. They have far better things to do than hang around saving childrens' lives. They need to be on Clapham Common making sure those lawless cyclists don't step out of line.