
Guest, no reputable breeder of 'teacup' yorkies exists, because all they are is undersized, possibly unhealthy ordinary yorkies.
Your best bet is to contact yorkie breeders and ask if they think they're likely to have any smaller-than-usual ones in a well-bred litter.
However dogs aren't suitable toys for children, and I don't think anyone will sell you a puppy for a toddler, particularly not a small, fragile breed which would easily get 'broken'. Why not buy your baby a stuffed toy till he's older?