It would be helpful to us if you would post the specific snippet of your code (in proper code-tags) that is responsible for launching the child process and gathering the STDOUT/STDERR streams from it.   Superficially, that ought to be simple ... just intercept STDIN as well ... but things in Tk are not always so simple.   So, let us see just how you are now launching the child.   And, confirm how you are recognizing that it has produced an output that is a prompt.