Breath, just Breath.
I have taught a lot of young ones to knit. Here is my personal approach. I am sure others have their ways too.
Little hand, shorter straight needles.
Bigger hands, standard straight needles.
Advanced younger, circulars.
Large needle size. Being that I am in the US I will say a size 12 or larger.
I cast on about 20 stitches for them. Then teach them to knit only. Getting them to keep the same number of stitches on their needles is sometimes hard. When it is long enough for them I cast off for them. First lesson done.
Next, I cast on 20 stitches again for them. Then I let them knit 5 rows. Next row I show them purl, then knit, then purl. You know SS stitch. After 10 rows I tell them to knit 5 rows, then knit 10 rows of SS stitch again. Continue the pattern for as long as they want. When they are happy with the length I make sure they have a 5 knit row last, then I cast off for them. Second lesson done.
Third lesson is a bit harder. Casting on. I show them how I cast on. We work on casting on for a long while in most of my lessons. It takes a bit for them to get the tension right. Once they have cast on the number of stitches they want I let the knit or purl as they want. Lesson 3 done.
Next time, they cast on, knit or purl as much as they want and I show them how to cast off. Most times my children are spending time just working away on their projects alone. So when they come back they are ready to learn something new.
Start with the very basic and you will be happy with yourself. Children want to learn everything NOW. They have to take it one step at a time.
Depending on how they have done for the first lessons is what I go onto next. It is fun to watch the start to get the idea in their heads that by mixing up knit and purl they can make some interesting patterns.
Good luck.