Today is the second day of class and the topic that I've learnt is on "Whole Numbers".
I find that the usage of Ten-Frames is very helpful in developing number sense in children. Research has shown that both children and adults have the innate ability to subitize - to instantaneously recognise the number of objects in a small group. Ten-Frames can help to organise numbers in groups of 5 and 10 to match the base ten-number system.
Once the children the system of Ten-Frames, they are ready to proceed to other mathematical concepts like addition. You may like to try this link to play games using the Ten-Frames.