Tried and tested within a school and daycare. 100+ children have learnt their numerals this way already.
Flash cards, come in A5 but can be scaled to required size. 0 to 20.
Learn your numerals with simple rhymes such as "zero to hero" adding in a described action. These pair up with a characterised numeral, that kids love!
Eventually just actions or rhymes can be used in everyday life as a hint or reminder.
Cards have been professionally designed with the numeral and rhyme on the front, and action on the back.