While is not necessary you can install gnome-font-viewer to preview the fonts, create thumbnails read their licences and easy install them on your system.
Open your browser, go to http://www.google.com/webfonts, select your collection/collections you like and press “Download your Collection” link on top right
After downloading, you just unzip them and you open it with font-viewer. If you didn’t install it, doesn’t matter, just copy the fonts in ~/.fonts.
Now open gnome-tweak-tool and select your fonts, that was all!
While Butterfly Kids looked like a nice choice, it turned out as totally unreadable font :)