If it's salary sacrifice pension, which many are, then throwing more into your pension will reduce the taxable income. I expect there will be many doing this to scrape under the 100k threshold.
I would imagine all the big earners will be getting nannies provided by work to compensate, or probably have them anyway, much easier than worrying about your own kids. They'll all be off to board at 13 anyway.
How many companies paying someone £100k will also give them a nanny? Sure there are companies who offer sickness nannies to cover a certain amount of time if your kids are ill, but if they gave you a nanny that would still be a BIK.
Think of it this way, the higher earners with nannies are able to work and pay income tax. You should be annoyed at people who can perform well paid jobs but stay at home to spend time with the kids :)
If it's salary sacrifice pension, which many are, then throwing more into your pension will reduce the taxable income. I expect there will be many doing this to scrape under the 100k threshold.