First, look away to respect her privacy. Then, in a calm voice, say something like 'Mom, I'm sorry. I didn't mean to intrude.' And then, give her space. If she wants to talk about it later, be open and honest about it, like explaining that you just happened to be there by accident and that you'll be more careful in the future.
Apologize sincerely. Try to have an open conversation with her about why you did it. Assure her it won't happen again and respect her privacy and belongings in the future. For example, you could say that you were just playing around and didn't mean any disrespect.
One possible story could be that the mom and son are at the beach. The mom is wearing a bikini and she is teaching her son how to build a sandcastle. They are having a great time together, enjoying the sun and the sea.
Since we don't have the details of the story, it could be a child. Often in such stories, it's a curious child who doesn't fully understand boundaries yet.
It is not appropriate at all. Such a story might make people feel uncomfortable, especially if it involves objectifying or disrespecting a mother figure. Everyone should be treated with respect, and we should avoid creating or spreading stories that might have a negative impact on others.
I'm not sure specifically what this 'boy caught in shiny bikini story' is about as the title is rather unclear. It could potentially be a story where a boy is found or gets into a situation while wearing a shiny bikini, perhaps in a comical or unexpected context.