The toddlers have two kinds of memories: things that happened yesterday and things that happened "westerday" or any time in further past than yesterday.
I think they found and filled a gap in the English language. We don't really have a single word for "happened a while ago."
As long as they are this little and perpetuate the usage between them, we have a valid new word.
Westerday. It's a thing. Get on it.