Miss FFJA is ‘buying’ a locket for her mummy for Christmas, does anyone know how I can go about having a photo printed to fit?

Just cut one out to fit?

Go to your local digital print shop with suitable photo on SD card and get them to print it off to the right scale?

Historically, people would have a print made from a negative, and cut the head out, negs tended to be fairly small, 120/220 roll film, and often a contact print would be fine.
A good digital print from an Epson printer, which uses archive-quality inks, sized to fit the space, should be fine.

Had same issue. Just printed off a photo of the babies face small, pressed it into the locket to give me a cut line then 2 mins with a fresh stanley blade. Looks decent!