I’m not sure. Imagine you want to answer the research question of how people come to adopt certain beliefs. You can probably answer that robustly by monitoring absolutely all their cultural inputs - including throughout childhood where these inputs likely have high impact, but where consent for such an experiment is likely impossible. Every book they read, every website they visit, every person they listen to. I think if you constrain this experiment to a more reasonable level of information gathering, you’ll miss important details.
You could get permission from the parents to install the recording devices, but until the child is grown up the data could be kept locked away somewhere completely inaccessible. Then when the child is old enough to grant permission you unlock that data for use. This sort of pattern should work for all studies involving children.
Honestly though, most of these kinds of studies would probably just stop at getting permission from the parents.
