You are completely correct with the minor edit that this study would never be shutdown as there is a 0% chance it ever gets approved.
Beyond how strict IRB/REB boards are in general, there are also additional federal requirements that must be satisfied when submitting a study proposal on children including surveys and not just interventions. In the US these are the pathways:
Section 404 requires that research poses no more than minimal risk. As the intervention itself is a federal crime this is not satisfied.
Section 405 allows for more than minimal risk if the anticipated benefit for the study subject justifies the risk. There is no individual benefit to the minor for watching pornography.
Section 406 requires that the risk is "commensurate with the subjects expected or actual psychological and social situations" which is hard to argue is expected when it is illegal to begin with. It also requires that the intervention is "likely to yield generalizable knowledge about the subjects’ disorder or condition which is of vital importance for the understanding or amelioration of the subjects’ disorder or condition", hard to argue that answering whether children viewing obscenity is addictive provides knowledge that is vitally important when there is no clear reason why children should be exposed to pornography.
If an IRB cannot satisfy sections 404-406 the only other way to get such a study approved is section 407 which requires direct approval from the Secretary of Health and Human Services if the "research presents a reasonable opportunity to understand, prevent, or alleviate a serious problem affecting children’s health or welfare, and, in addition, determines that the research will be conducted in accordance with sound ethical principles and that adequate provisions are made for soliciting the assent of children and permission of their parents or guardians". It is impossible that this would be satisfied as children NOT watching pornography (the alternative treatment/status quo) is hardly a problem let alone a serious one.
This has nothing to do with religious justifications but is due to very reasonable and heightened ethics requirements when conducting research on children, be it pornography or a new medication. Canada is essentially the same and I can't speak to Europe but I imagine most countries have similar requirements regarding to needing to demonstrate likely benefit to research subjects or contribute to understanding of a disease (which is nonexistent in paediatrics).
Every researcher thinks their question is relevant and important and that's why we have these rules in place.
https://www.hhs.gov/ohrp/regulations-and-policy/regulations/...