By acting as a man-in-the-middle with the ability to read unencrypted message data (absolutely required in order to try to match against known CSAM), this is absolutely providing a backdoor as well as undermining privacy and security. By needing to trust another party, there is now a greater threat surface which is outside of end user control. One compromised account with access to that third-party is all it would take to extract private details from any messages, undetected, whether for sale on there blackmarket or for suppressing political dissidents, that's exactly where this would go and we know this because state actors have been caught doing it and getting their toolkits leaked to criminals.
This kind of law doesn't make children or regular people any safer.