Focus on one form of writing at a time. Japanese features two syllabaries, which are used in place of the roman alphabet when writing sounds. In English, each symbol represents a portion of a sound found in the English alphabet. Japanese is based around complete syllables, which are the combination of a consonant and a vowel sound. Hiragana is the syllabary used for native Japanese words, while Katakana is the syllabary used to write foreign words. Katakana symbols are a good choice for introducing children to Japanese writing. The symbols are less complex, and they are used to write and pronounce words borrowed from other languages. This means that "Merry Christmas" becomes "Meri Kurisumasu." Using borrowed English terms can help teach children how to convert English words into Japanese, and will provide a basis of understanding the language using existing terms.
Break down the syllabary into smaller portions. Focus on just five characters a week, and assign writing exercises so that kids have reinforcement while memorizing the symbols. Japanese is organized based on combinations of consonant and vowel sounds. Focus on one row of consonant sounds for each week to teach kids a natural progression. For instance, if you were working with the "K" sound, you would teach "Ka, Ki, Ku, Ke and Ko." If you were working with the "R" sound, you would teach "Ra, Ri, Ru, Re and Ro." Breaking down the syllabaries into smaller portions will help the kids fully grasp each symbol, and will help them with long-term recall.
Utilize mnemonic devices to help kids memorize difficult symbols and phrases. Focus on how each symbol resembles an image or letter familiar to native English speakers. An often-used example is that the Hirgana symbol for the sound "Fu" looks like a mountain, so you can remember it by thinking of Mount Fuji. There are a lot of freely available resources for teaching Japanese using mnemonic devices, so try to pick some that are age-appropriate and pertinent to the teaching style you are using.
Assign regular writing exercises for homework. It is very important that kids learn to write the Japanese symbols in a consistent and quick manner. Writing exercises will increase their penmanship and help them memorize the many different symbols.