Emma Hey congratulations on your class! Sounds like you did great
45 minutes?!? Blimey, that's a lot for a 6-year-old, Were they sitting with the guitar the whole time, or did you do some kind of moving around? I think it can be hard for a young child to sit for a relatively long time, and I find holding the guitar as well makes it a bit more tiring too. Most of the younger students I've had kind of start swaying about in their chairs or turning the guitar upside down and stuff after about 20 minutes or so. The 6-year-old I've got now does well compared to most kids I've seen, but still 30 minutes is enough for her. I have a 7-year-old as well who has the lesson late in the day after school and gets a bit tired, so 30 minutes is very much enough for her.
I'd break it up with some musical/guitar games in the middle. I don't have any great suggestions, but I wouldn't necessarily worry too much about them not being on the guitar, you can play for them and they can react in a certain way, or vice versa. Like you play fast/slow, they run around fast/slow, you play high notes, they walk around on their toes, low notes, the crawl around... just playing around with the parameters of music.
Regarding posture, if you take Eduardo Inestal's advice... I don't know if you saw the two live streams he did on teaching (I HIGHLY recommend them, they were essentially what pushed me over the edge to feel confident enough to teach), he basically punctuates his discourse of sage advice with telling you to always check POSITION. Personally I always do, but I also always do what I think they can manage. I also make sure that they understand why I telling them to sit in a way that is apparently uncomfortable is good, for example, I ask them how it feels to change how their hand is, is it more secure or unstable, does sitting up straight make them feel sleepy or more alert and ready to make music... stuff like that. Posture is basically a very complex thing that's made up of many smaller considerations and is different for everyone, so one thing at a time, and what they can manage without demotivating them at all.
One thing for example is that often there is a tendency to turn the guitar up and turn the head and body to the left to look at the left hand, which is obviously bad posture, and if they play like that forever it would be far from ideal. But, I think it's reasonable for someone to want to look at what they are doing, where their fingers are etc., so I think as long as they understand what the "centre" is, it's OK.
My youngest student (6) plays with the guitar on her right leg, on a low chair (where her feet reach the floor) and a low footstool under the right foot. She's more comfortable and I find her arms and hands are in a better position that way. That might change later though.
Cripes, I think I've blathered on enough! Good luck for your next lessons and keep us posted!!