This only makes sense if the plan is, in direct conflict to what was just specified, to do something like scan all messages for content like CSAM (aka child porn). Then the need for the decryption keys does make sense.
But, if you accept their claim that this power will only be used rarely when they have particularized evidence that there is some kind of criminal conduct or threat it sounds like it will backfire. For this to happen, someone has to report their kid is getting disturbing/suspicious messages and whoever does that reporting can just hand over the received message to the government. At that point, mainstream encrypted messaging apps still allow the government to get the sending phone number and they can go serve a warrant on whoever has that phone (even if it's a burner, they can just get a warrant for it's position). Even if you argue that it's necessary to see what that person has sent to other individuals you have their phone so you get all the messages still kept in the history (and even if you have a decryption key that doesn't let you rewind time to undelete deleted messages after the retention period has past).
OTOH, if they pass this law people are likely to turn to other options such as sideloaded apps on android or computer based apps that might use a phone number to prove identity. Unlike meta, apple etc it's quite plausible these apps either won't collect at all or won't expose the true phone number to the person getting the messages (and overseas servers can make it legally difficult to recover that information even if present).
So is it really about protecting children or are they lieing out of fear that adults will get away with crimes and they won't be able to gather the evidence? For that application, it makes sense to worry about how hard it is to access messages that people send on convenient messengers which neither party is willing to share. After all, the police might discover a bank robbery because the bank gets robbed.