Young children develop subtizing skills much like they learn to read sight words. The ability to take a mental snapshot of letters or objects and know what the number or word is comes because of opportunity, exposure and reinforcement. Eventually young children learn to recognize the number of small groups of objects simply by sight (we call this subitizing). Using common patterns of object placement, such as the ways pips appear on the side of a die, children recognize a group of 2 or 3 dots and eventually, without counting, they know how many there are without counting the pips themselves.
Just like frequent opportunities to read sight words support the development of the skill, so too, do frequent opportunities to subtitize number. In addition to having several kinds of dice and dominoes in the classroom, other materials with small numbers of objects should be available so children can practice. Begin with groupings of 1 to 5 as these will provide the foundation for subitizing larger groups using part/whole understandings as well as composing and decomposing number.
This memory game has small groups of objects with matching numeral cards. Start with 1-5 and put the rest away for later.