This is the exact situation I am in as well. The only difference is that I have three kids, and there is only ONE child this happens with, and it doesn't matter if I try to access communication limits and try to put in the correct screentime passcode. It also happens if my wife, who is a parent/guardian on the family, does it from her iPhone. Also only with that one child. The only difference between this child and the other two children is that the one that isn't working is using an apple ID she has had for over a year now, and on a brand new iPhone, and her apple ID has extra iCloud storage.
My phone, her phone, all updated to latest iOS. Both phones have been restarted.
I am doing this as the Organizer account. To be clear, one moment I see the settings app screen where I'm trying to access the "During Downtime" area of "Communication Limits" section of "Screen Time" for the specific child, and the next moment WITH ABSOLUTELY NO ERROR MESSAGE I am dumped onto the home screen. If I go back to Settings by opening the currently opened tab (the tab for settings IS still open), it brings me to the initial Settings page (Account, Airplane Mode, Wi-Fi, Bluetooth, etc.). If I try again, and again, same result. I have tried force closing the app after it does this, and resuming the app after it does this, trying again... crash.
I have read online somewhere someone else with our exact same issue, and they went one step further, and created a whole new Apple ID for their kid and added it to the family, and it fixed the issue. I however, hardly call that a fix, as I'm currently paying for extra space on my daughter's iCloud storage, and I want that to carry over. I looked into removing her from the family and re-adding her, but that is a whole ordeal where apparently I have to add her to a different family first, because she must always be a part of some family, then re-add her to the real family. I may be at the point of trying that, but I feel there should be a simple fix somewhere. Maybe it's a glitch with the latest iOS version not playing well with something?
I have been Googling for an answer, and so far, NOTHING aside from that, and every so often another post from someone like you that has not found an answer yet.
Let me know if you find something out, please!