At ten you are being ridiculous. He's not three.
A couple of points about your ridiculous logic.
At 11 most children will be walking to high school by themselves. You need to get them into the habit of doing things by themselves as preparation for this.
DS is going to high school in a few weeks. Some of his peers are still ten. It's insane to still be insisting that a ten year old boy is never out the sight of an adult. I'm assuming this kid is actually year 5 about to go into yr6 though, but even then it's over protective really. Half of yr6 is about preparation for high school not actual academic school work. It's totally age appropriate to be letting him off on his own within an enclosed area.
A maze is a contained space. It's going to be full of other families. It's not going to be a case of no one around and being completely alone. The kid was happy to do it on his own. A maze is also really not that big. And people have voices.
How would a phone have stopped a sexual assault? This is the thing I ponder with insane over anxious parenting. They think that phones will somehow keep their child safe. And don't think about how it might get them mugged or be fucking useless if something were to happen. The phone is really a blankie for the parent. It allows them to project their anxieties onto children. Kids really need to learn skills of how to cope in a crisis if they are separated from their phone / their phone battery is dead. Because that's actually the most likely scenario. A maze in a park is actually a really good place to practice these skills of coping alone and having a strategy to cope.
We have a wave of kids with anxiety disorders and we blame a whole pile of things for it. Except mad parents who don't teach independence and plan to gradually let go at age appropriate times because they want to infantilise their child and don't want them to grow up.
Kids have to cope. You can't protect them from everything. They need to learn to deal with everything from imaginary monsters under the bed to how to cope if something does unfortunately happen to them. Otherwise we just lock them up inside and they never learn to live a normal life anyway. It's your job as a parent to guide them through this process, not to make them incapable due to your anxieties.
Kindly, you need to look at how your own experience is still affecting you before it impacts on your own child. Get some counselling. This isn't about your DH being naive.