Similar issue. My son is trying to use an app that I had not prohibited. His Safe Family app pops up when he tries to use it saying that I did prohibit, and to ask permission. He does this. I get the request in my parent app. I click to "allow" and various time choices...forever, 30 minutes, 1 hour...this is where I get the 1003400 error message. This only happens with one app on my son's phone. Also, as with the OP, I cannot see the list of my son's apps. I have an S7 running Android. He has a J7 also running Android. Thanks.
We are aware that some people, ilke yourselves, are experiencing this issue. We are working on fixing this issue in our upcoming update.
in the meantime, the way to work around this issue would be to set up a new child profile for your kid and then "switch profile" on your kids device to the new profile so you can link the device to the new profile.