In the US, COPPA requires that tracking kids or even storing most PII of them be accompanied with parental consent. YouTube got hit with the biggest fine to date for doing this, partially because kids were watching kids' YouTube videos but on their parents' google account, and thus were being tracked and getting ads targeted towards them[0].

However, this research seems to be about children clicking ads, and then being tracked by that third party (or maybe even google ads/doubleclick on the page).

0: https://www.ftc.gov/news-events/news/press-releases/2019/09/...