This shows an ignorance of how law enforcement operates in this space. Sometimes it is retroactive as you say, but often it is preemptive. For example, cops sometimes go looking on social media platforms for people soliciting CSAM, strike up a relationship, and talk people who take the bait into a physical meeting, at which point they pull out the badge and a warrant and examine the target's phone for evidence of existing possession. Another approach when potential crimes are reported (eg by a parent who discovers someone attempting to seduce their kid) is to silently take over the victim's role in the conversation and lure the would-be predator into a meeting, at which point the cuffs come out.

Anecdotal example, but this is seems to be the opposite of wanting to "protect children"

Yes, but what of it? This is a fallacy of composition, which assumes that failures or perverse legal outcomes are all part of some twisted masterplan designed to strip away freedoms. It's just cope, like the arguments in the wake of mass shootings that they are all secretly engineered by the FBI, based on the fact that the FBI or other law enforcement agencies sometimes fail to act on warning signs that seem obvious with hindsight.