Flexibility of thought difficulties include struggling to handle change, it is easier to process that things are the way they are and will always be that way. It is difficult to process when plans change and you have to suddenly think about something differently that you had already planned for your thinking for. So if you plan a day out, planning what train to catch, what you will be doing, where you will be eating, etc., then you want and expect the day to go like that. It can cause anger and anxiety when things don't follow this pattern. So if it is decided not to go out now for the day then this can be difficult to handle, suddenly there is uncertainty about how things will go, and it feels like the plans were just a lie because they never happened in the end.
It is difficult to interpret and understand the thoughts, feelings and actions of others. I know most people find this at times, but in most situations most people can grasp why someone is thinking, feeling, or behaving the way they are, whereas those with autism struggle to do this. If someone is crying they may be able to work out that the person must be upset, but they may not understand why the person is upset. This can lead to seeming insensitive or saying the wrong things. It is easy to accidentally offend someone because you can't connect how they are responding with any cue, and this can even be with things you have said. So you may upset someone, they are now crying or angry with you and you can't figure out what has just happened to make them cry. And the way people seem to communicate doesn't help, because often the response is that you should know why, and should know what you have done - but you genuinely have no idea why or what you are supposed to have done.
When I was younger I used to find it very difficult to understand how things were going to go. I hated (and still do) uncertainty, I would avoid situations that didn't have clear patterns which made them predictable - this often meant avoiding situations with people, because even seemingly predictable situations can become very unpredictable once you involve people. As I have grown up I have learned to do mental rehearsal. I will spend hours mentally rehearsing situations and how they are likely to go, and I will practice multiple different outcomes which could occur and how I will react in all of these different outcomes. I will also practice very unlikely outcomes and think of what I will do in these situations. This way many things that happen that I had time to prepare for like this I know what the outcome will be which helps me handle these situations better and reduces the uncertainty in the situations. This is why I find it easier to handle significant incidents - like someone attacking me, or an emergency, than small things, like interacting in conversations with people, or catching a bus somewhere.
Many people with autism struggle to recognise danger. As this is something I think most children and teens have it can be difficult to identify whether not recognising danger is linked to perhaps having autism, or just a normal part of growing up. There would be other traits you would also be looking for as well as the child not recognising danger. I personally used to be aware of dangers and would calmly deal with them, but was never bothered by most dangers. As I have grown up I've become even more aware of dangers due to the type of work I've always done. I suspect I've always been unaware of certain types of potential danger, although I'm more aware now. So if I could see that something was likely to cause a fire, I would go and deal with this. If I was babysitting my brothers as a child I would recognise if they were possibly at risk, but I wouldn't feel intimidated around people so I could enter situations which could be dangerous and not notice, or I could do things which I feel wouldn't be a problem - like putting metal knives into toasters to get toast crumbs out which were touching the elements and create an electric shock. Friends have often felt protective over me for example, when I will have no problem going to a party where others feel would be dangerous parties to be at. I am always told I'm too trusting of others, and believe what others say too easily.
Another area of flexibility of thought which autistic people struggle with is often imaginative play or activities. For example, they will often build a lego model of exactly what the box says to build, and are less likely to build a model of something they make up, or they will play with a train as a train, and will struggle to see that the train could be a gun, or an airplane, or hammer for a game. With practice this skill can be developed. I don't know how most people think flexibly and imaginatively about play or activities, but one way I've found to work is to think 'what can this be used for?' and try to think of similarities. So a train is long and thin - rockets are long and thin, so you could pretend that the train upright is a rocket. The train is long and thin, hammer handles are long and thin, so you could pretend that it is a hammer, the train carries people and is long and thin so you could pretend that it is a weird airplane, the train is long and thin, the nozzle of a gun is long and thin, so you could pretend it is a gun.