That works if you need to communicate with them one-to-one. But does Signal let you easily add any other Signal user as a contact? What about when your kid wants to talk to their friends on the app?
To me the appeal of what Facebook is doing here is that it lets you give your kids freedom to use the app while letting you as a parent create the whitelist of who they're able to talk to.
Signal works with phone numbers, so you can talk to any contact that is also on the app. There is no parental control but how do you assume kids can't text random people. Blocking/banning makes users highly creative.
To me the appeal of what Facebook is doing here is that it lets you give your kids freedom to use the app while letting you as a parent create the whitelist of who they're able to talk to.