Where have all the children gone ?
I believe this question will be asked many times in the future by people studying photographs from around the beginning of the 21st century.
If you look at the work of many of the great documentary/street photographers like Henri Cartier-Bresson, Vivian Maier, Dorothea Lange, and from here in the UK, Bill Brandt, Oscar Marzaroli and Joseph McKenzie, you will notice that children feature significantly in the body of work that they have produced. They have given us a great window into the lives of children in the mainly post-WW2 era and through to the 1980s. But then life changed.
With the advent of the internet and camera phones providing the capability for anyone to make and share images worldwide almost instantly, people began to become more guarded of their personal privacy and especially that of their children. The media was also highlighting the dark side of the internet, especially the trade in inappropriate images of children. Their innocence was being lost and replaced by them being a tradeable dark-web commodity.
Although it is not illegal to take photographs of children in public, it is illegal if these images are of an indecent nature or indented to be portrayed in an indecent nature. A lot more guidance can be obtained from the NSPCC website.