Otocinclus catfish
Corydora catfish as mentioned above
And tetra's CAN be tankmates, but this can turn sour. Unadvisable unless your nephew knows what he's doing.
I think it all depends on the temperament of the particular fish. Mine was kept in a community tank with a variety of other species, but I've read of other members who have had examples where they could only be kept on their own.
ive always managed to be able to keep mine with a mixture of neon tetras, mollys, platys and guppies without any trouble.
but like someone else said what has worked well for me may not suit your nephew.