I don't know why is it so hard to find info about this, it's as simple as get in a text file and change what is inside. But somehow, I can't find this file, I looked everything related to nvim, with a CTRL+F looking for "poor children" but I can't find it
I just want to be able to change it without any extensions
That message is embedded in the binary, you can't change it by editing a stand alone config file. If you really want to change it, change the source code and recompile it.
Another janky approach is to change the binary, use "grep -irn <nvim_path>" to check if the string exists in your neovim binary. It exists in mine. You should be able to replace those characters with spaces, finger crossed it should still run. (Adding extra characters will mess with binary.)