Not a guitar teacher here, but violin. Which does mean that 8-year-old beginners are the norm for me!
Forget about how you structure an adult’s lesson. Forget about sitting down for a whole half-hour. Forget about even holding the instrument for perhaps any more than half of that time, to begin with.
You’ll want to do some basic musicianship work, probably with an emphasis on rhythms, and certainly on physical coordination. Think of it as a more all-round music lesson rather than just about the guitar. Clapping games, copying rhythms, using word rhythms (foods they like/don’t like is always a good one). How to remember the names of the four strings on the violin? ‘Every Angry Dog Growls.’ Which one’s the lowest sound? It’s the GRRRRRRowl, of course. So growl like an angry dog. Lose your inhibitions, and make it into games with a purpose, rather than simple instruction. With boys, you can often get good reults from having them gently compete against one another. Who can copy the more difficult clapping rhythm correctly. Marks out of ten for good singing. And so on.
Ten minutes of this, perhaps, and you haven’t even got as far as touching the instrument…
Introduce a few technical aspects and terms/names each week, and keep reinforcing them. Naming parts of the guitar, again you can have a competition of who can identify them first when you point to them. Add one or two each time, from the most basic ‘neck’, ‘strings’, on to the detailed ones.
As for technique, take the basic adult description, and turn it into something that will be memorable for a child. Examples I use:
To an adult beginner: ‘The first step to form a bow hold is to take the thumb of the right hand, and touch the tip against the inside of the middle finger, in line with the end knuckle.’
To an 8-year-old: ‘Which one’s your right hand? Isn’t that your left? That’s right, that one makes an L, so it’s the other one. Now make an elephant, like this. Make sure he can wiggle his trunk!’ Later on the elephant holds a pencil in his mouth (but don’t let him swallow it!), falls asleep so his ears flop down beside his trunk, etc.
To an adult: 'The upper arm should be aligned with the bow, so when on the G the elbow will be up as high as your head, and when on the E it’ll be close the vertical.
To an 8-year-old: 'Put your violins down in front of you. Be a chicken! (Yes, you become one, too!) Flap your wings more, like you’re going to take off. Now chop one of it’s wings off.’ Always gets a laugh out of them, which means they’ll remember it. Then later on, when actually playing: ‘Remember your chicken’s wing? Play G. Now E. G. E. Don’t let your chicken get lazy!’
Others will be able to point you towards instrument-specific resources, but this is a good site to browse through user-created classroom music activities, to get an idea of the content and the level to pitch it at for the age range (8yo= Key Stage 1, Year 3 or Key Stage 2, Year 4), and also to see the structure of lesson plans. This is another one, this time mainly pointing you towards professionally-supplied content, but with some free stuff available.
Use a simple lesson record book, which I restrict to a few basic reminders of what to practice, that they will be able to understand by themselves. You’ll be disappointed if you expect them to be able to remember everything from one week to another, even if it’s written down. Communicate with parents if there’s anything more complicated or specific to be checking on. (I’m not a fan of involving parents in lessons. It goes without saying that kids are completely different when parents are not around, and this is almost always a good thing.)
One other thing, which is all too often overlooked: part of a music education should be about listening and appraising. Once they’ve got a little way on, they should be able to listen to each other play, and to make comments both on what they’re doing well and things to be improved. Even more important is to perform for them every so often: too many kids never actually get to watch their teacher play a real piece, beginning to end. To be able to see a ‘real guitarist’ playing, live and close-up, is most likely something they won’t have anywhere else. They probably will have questions to ask afterwards, as it will feed their imagination and interest in what they’re going to learn later on.
And finally, don’t expect every lesson to go as planned. Kids are kids, there’ll be days when they’re tired, or just not able to concentrate. Be flexible, too. If one of them is completely distracted because it’s his birthday tomorrow, have him think of what presents he might be getting, and clap the rhythms of the words for the other to copy, and to try and work out what the words are.
Oh, one extra thing (this is rambling a bit, now, isn’t it? 
): stickers. Children can’t get enough of them. Relieve Staples of their shiny stars, smiley faces, ‘Good Work’ thumbs ups, and whatever else they have in stock.
Have fun 