Can’t go wrong with a Tanglewood EB -18 Elfin bass. Around 20 inch scale IIRC, looks like a shrunk Precision! They play and sound really well, and can be picked up for around £80 or less if you’re lucky. Ideal for small hands, but I’ve used mine on a gig and it coped brilliantly.
Think Harley Benton do a decent short scale, the PB shorty, currently £79 at Thomann. More of a ‘normal’ short scale at 30 inches so may be a bit big for your nephew just yet. Good luck!