Good goggles over glasses for a small head?
Safety Glasses.

I no longer bother with prescription sunglasses, I have multiple pairs of Uvex safety glasses that fit nicely over my glasses.  There are multiple lenses that filter all UV, come in different colours, etc.  I also have a pair with clear lenses that I use when it's particularly dusty or dirty.  (e.g. I sometimes ride over a very busy bridge)

If you do go with some sort of safety glasses, you can either find someplace online that sells them or look for a local safety/industrial supply company.

Here's some more Uvex-specific information, I'm sure other brands would also work:

The glasses I use are the Uvex Genesis with Espresso Lenses.  The Gold Mirror lens looks cooler (both are 100% UV) but the gold surface finish is more fragile.  The lenses on these and many other safety glasses are interchangeable so if you want another colour or need a replacement, you don't need to buy the frame.  OTOH, the frames are not expensive so it's possible to have multiple shades for different conditions.  (The last pair I bought last year was about Can$35.)

Uvex has a smaller series called the Genesis Slim that might meet your need for a smaller pair of glasses.  They don't have as many colour choices for lenses, but both of the "Grey" choices block 100% of UV.  This page has PDFs of reference info: which tints are available for which frames, details of the different lens tints.