Unless this toddler is a more calm and gentle child, I would keep them separated whenever possible. Most (not all) toddlers have sort of erratic behaviours, like sudden screams/laughs/bursts/zoomies, and that usually freaks out Yorkies. And if it's only three days, it's not really worth the effort to get them accustomed to each other...
But yes, if they do
have to interact, I'd keep him on the leash. And make sure the parents are willing to teach the toddler to follow some ground rules as well. It's just always harder when it's other peoples' kids. Those without dogs sometimes find it hard to understand our worries and when/if something goes wrong or the toddler cries, it'll be the dog's (or your) fault
