Consider "mandatory reporters". People like schoolteachers are legally required to report certain things like parental abuse or neglect of their students. If the government wanted to write a law that said you are required to report a crime if you witness one, I'm not sure that would be unconstitutional.
As an aside, if you are legally required to report something you have seen, and do not, then you are violating the law, which means legally requiring you to report yourself is unconstitutional under the 5th amendment, so we enter into paradox territory...
As they said on slashdot 100 years ago, child porn is the root password to the constitution. Configure sshd to only accept keys!
For example, a layperson can ignore someone having a heart attack but a doctor is required to help (even if it’s just calling for an ambulance).
I understand the government's interests in this particular issue, people that abuse children are the biggest pieces of shit that I can imagine, but unfortunately math is a pain in the ass and their laws are not possible to administer.
If you think someone is abusing children, you can always send a cop to their house and have them check. That is well within the rights of the government, and I'd even go so far as to say I support that right.