[Columbine]

I do the exact same thing at my local school. Forget about writing characters; trying to teach 90 'kana will take literally ALL your time, and they'll forget it in a flash. GIVE each child the katakana for their name (or a katakana chart and help them find which symbols are 'theirs') and let them learn those specific ones. It's such a short time for you to be teaching in, you'll kind of have to swallow your pride and use romaji. I normally never would, but in this kind of situation it'll get so tedious and frustrating for everyone if you don't.

Our kids find the language parts really tiring, so we only do about 20-30 mins of language to start with, and then wrap up with games or crafts based on something cultural. We've done vocab bingo, and ball games with numbers, which they really like too. Also, LOTS of drilling. They're good at mimicking, but remembering the words is hard for them do they have to hear it a lot. Usually I model a word before they even see it, and get them all to repeat it a few times. Then they can see it and write it down. We give them model dialogues too, usually the lesson after they've learnt the vocab, so they can practice putting the words in context, and swapping them in and out. And if you planned a quick language exercise and they seem to be focussing well, stick with it! If you've got their attention and they want to work on something, might as well roll with it.

We've done numbers, time, colours, animals, national holidays, myths and legends (their favourite, we get asked about that a LOT), painting (with vocab), nationalities/countries, history, tokyo, the emperor and his family, food (also popular), japanese playground games, greetings and classroom japanese (sometimes I do this as a language blitz, i model 'konichiwa' with actions and get students to say it to me until they realise it's a greeting, then I add in 'my name is~', 'what's your name' and 'pleased to meet you' in stages, getting them to work in open pairs, then mingle. It's potentially confusing, but if played out right, can be really fun and they learn masses in a short space of time.)

good luck, it's loads of fun and the kids enjoy it!