I have named changed for this.
Last summer, we were all happily sat in the garden (Me, DH and two teen DSs) when the police turned up at the house. Two officers.
They had information that illegal child pornography had been viewed in the house. They were actually already pretty sure which phone it was on (thankfully since if they had thought it was DH it would have been far more difficult a situation).
It transpired that DS2 who was 13 had been sent pictures on snapchat and via discord. He was going through a period where he had made online friends (initially through minecraft) and as part of that he had been sent photos.
The police went through all the DCs devices. They literally stayed in the house and went through their laptops iPads and their phones. They were here for a few hours.
They found a number of pornographic images which had been sent via snapchat from randoms (of adults trying to encourage dialogue with DS2). They also found images and videos of underage children. Plus evidence of hensai (japanese cartoon porn) on DS1's device.
In DS1's case (he was 15) they warned him that hensai is still pornographic content and because there is a tendency to portray characters as much younger that this counts as child pornography. They said if he was older they would still be able to prosecute over possession of this (although the police officer told me afterwards that there wasn't much on there really so it was more to give him a scare).
In DS2s case they said that the stuff on snapchat was of underage girls but that because the girls were of DS2s age and it was clear they had sent the photos and videos to him unprompted that they wouldn't take that further even though technically it was an offence. DS hadn't forwarded anything on or anything like that. There was one image on his phone which he had deleted. It was a graphic image of a young girl probably under ten and DS had responded saying something along the lines of "FFS don't send that to me - way too young - just delete it". The police made it very clear that just receiving the image was an offence.
Both DC were in shock. When speaking to the police alone we told them not to pull any punches with them so they spoke to them at length and frankly terrified them both.
I actually felt like I was in shock for a couple of days after seeing the images.
Both DH and I work in a professional role where we cannot have any criminal convictions. If they thought we had downloaded the images we would have lost our jobs and been struck off so that we couldn't work again.
We then then had follow up calls with social services. Fortunately we were "discharged" pretty much straight away since they could see that we were supportive of the police actions and that there wasn't general reason for concern but we are now as a family "in the system" and if anything like this happened again with either child then there would be issues.
I actually think we were lucky. If the DC had been older the police would have taken further action. So I'm sort of glad it happened in a bizarre way. At the time it was absolutely horrendous.
Keep an eye on your children's devices. The crap about their right to "privacy" is outweighed by the damage that can be caused if you don't monitor what is going on (at the time I thought I did but I'm not familiar enough with the apps and so didn't know where to look).