Ordinarily, if the fonts are visible in Font Book and activated, then they should appear in Pages too. Pages is a font spectator, and you cannot add fonts to Pages itself.
Boot your Mac into Safe Boot mode by pressing and holding the shift key at the chime ... until you see the progress bar. This boot mode is slower, and may introduce non-harmful visual aberrations during the boot process. Among other things, this special boot mode will rebuild the System font cache, and that should allow Pages to see these Chinese fonts. Reboot normally, and then check again to see if the Songti and Kaiti fonts are available in Pages own font menu.
The other thing you may need to do is in System Preferences : Language & Region — add the appropriate Chinese Preferred language, and in the Keyboard preferences panel, add the appropriate Input Source language.