Use adding techniques that are understandable to these young kids, such as, but not limited:
Manipulative method-Use objects to demonstrate how addition works. Children respond well to visual tools that help them understand addition concepts. Any easily handled object may be used, from beads or blocks to Cheerios. Start with a small number of items and use a variety of tactics to demonstrate number relationships:
Give the child two small groups of items -- say, a group of two blocks and a group of three blocks. Have the child count the number of blocks in each group.
You can also use games that involve adding of objects, thereby making them having fun whilst learning.