I would put a capital A and a small "a" on one side of the card, and an example word on the other side. Printing them at home is probably the cheapest way to do it, on stiff card. Then you can laminate them if you like to make them last longer.
I'm not sure what you meant by "another side is printed with a diagram". A diagram of what?
I would also recommend printing out the entire alphabet in the right order, in both upper case and lower case, and perhaps sticking the letters in the right order on the child's bedroom wall (if it's not your child, ask the parent to do so). I assume the child is from Hong Kong so you have the added difficulty of a completely different script and alphabet.
Make them colourful and fun to keep the child interested.