bash: cd to the most recently modified child directory

Assuming your directories don't contain any unprintable characters:

cd  "$(\ls -1dt ./*/ | head -n 1)"

(note that it will also consider symlinks to directories).

If you don't mind switching to zsh:

cd ./*(/om[1])

Or to include symlinks to directories as in the first example:

cd ./*(-/om[1])