Even if you set up keyword blocking and number blocking via the message app settings, these blocking settings will not work if your child sets KakaoTalk as the default messaging app. The feature that allows setting KakaoTalk as the default messaging app is called "View messages together," and to prevent access to this menu, you can enable the "Block message settings" feature.

The steps to block the "View messages together" setting are as follows:

Once the rule above is set, access to the "View messages together" setting within KakaoTalk will be restricted as shown below.