No, I do not think that Early math teaching can harm child. Even i have hear that, Preschool Math helps children to grow quickly. flash card is also very useful for kids.

I hope i help !

flash cards are okay once the child has mastered the concrete method of doing math.  perhaps you could use m and m sweets to teach more and less, one more and one less, a basic bar graph to show which colour sweet is the most / least in the packet. flash cards consolidate concrete concepts.
hope this helps