I want to study about a toy design and optimization, how can design toy and teaching something by toy to kid's, in fact what's the best design of toy ?
I have no idea what kind off research you have already done. However, in terms off desk research a good place to start is by reading up on emotional design and cognitive ergonomics.
Ergonomica in general are of high importance in design but with childeren this is taken too a whole new level.
The good news for you is that there is a lot of research already done on this subject.
A link you could start with https://books.google.nl/books?id=YzMtzvs9sBMC&pg=PA495&lpg=PA495&dq=cognitive+ergonomics+child+toys&source=bl&ots=GZddxhRJPl&sig=q3NEGtmiu-eLNJQ0FxEzPlfNwVU&hl=de&sa=X&ved=0ahUKEwig2Y6W-ePXAhUFBcAKHX_5A1UQ6AEIWTAM#v=onepage&q=cognitive%20ergonomics%20child%20toys&f=false