No to both.
Was the psychological effect the result of repeated activity?
Were J&S’s alertness improved?
That first question is too hard to answer any better than “yes and no”. You can re-ask it in parts or clarify it if you want.
Did John and Susan use/have the toy as toddlers?
As elementary school kids?
As middle school kids?
As high school kids?
Is the toy a game?
Is the toy used for building or creating something?
Is the toy still being made?
Is the trait a mental association of some sort (like Pavlov’s dogs associating the bell with food)?
Is the trait something which the creator of the toy expected children would develop?
Is the toy stereotypically associated with one gender?
Does the trait relate to how they perceive the world?
Is the toy a model of some non-toy thing?
Is the trait they have in common related to music?
Is the trait a type of synesthesia?
NO
We need more information, still. See if you can figure out the rest. We are on the way, though.
Does the synesthesia relate to colors?
If yes, do the colors relate to
sounds?
letters?
numbers?
Is the toy used for assembling multiple times? (like Lego or a puzzle)
If not, is the toy used for assembling once? (like a model)
OK, Folly’s questions and answers should make it easier. What do you think is up?
The toy had letters of the alphabet in different colors. Perhaps cut out letters. Perhaps the cut out letters are matched to indentations in a board as a shape puzzle.
Aside from the specific form of the toy, the exposure to the specific color association with the letters led to the synesthesia?
Were they magnetic letters that you might stick to a fridge?
Yes.
Yes.
Answer:
Scientists discovered that while 1 in 10,000 have synesthesia, over 6% born between 1979 and 1990 had identical synesthesia, where the letters of the alphabet had, to them, the same colors. It was found that these people had a certain Fischer-Price magnetic letter set. And their synesthesia lined up with it.
It suggests part of synesthesia is learned.
I tried my best to make this puzzle getable and satisfying. I hope I did.
My very first response was a guess along the same lines, just different components. 
I noticed. Great first guess!
It needn’t be that early in life, either. I still have a touch of color-number synthaesia from the programming class I took in 7th grade, where the colors for graphics were coded by number. Of course red is 4, and green is 7.
Is it food or weapon? Most people would say it is the former but many experts think it is instead the latter. What is it?
Caffeine? (IIRC it is believed to have evolved as a toxin protecting coffee plants against herbivores).
**No **
But I like the line of thought!