I started teaching about a year ago and have learnt many many things since that time. It is a good opportunity for you, and you are right to take the responsibility seriously because people will do what you tell them and will learn bad habits if you teach them. However, this should also make you question your own aikido and delve more deeply into what you do and why.
Here are the main things I learnt:
1. don't teach what you are thinking about at the time (unless they are advanced students). Beginners really just need to know and practise the basics - don't rush into complicated things, just take it slowly with plenty of repetitive practise.
2. Don't try to cover too much in a lesson. Beginners find even one technique novel, and it is often difficult for beginners to get used to using hands, legs and body at the same time.
3. develop into the techniques slowly i.e. start off with excercises which train the body into a certain movement, then build this up until gradually a technique appears (this stops them having to think of a million things at once).
4. beginners will generally have a lot more respect for your teaching than you do (whatever the age) because you can feel/see your errors far more (again, beginners usually look at hands and feet).
Hope this helps, and good luck - it is worth all the effort.