Welcome Lolo
That is a tricky example and you're right, there are so many shades of grey with this sort of thing. You can never know what someone might sue over; they might be perfectly happy to have a novel written inspired by their story or they might blow up over a minor detail that you didn't think could be important to them.
A lot of fiction is inspired by true events. Some follow the facts very closely, others use the event purely as a jumping off point for studying a certain element of humanity that is highlighted by the event. It sounds to me like what you might have found interesting was not so much the actual news event, but rather the psychological impact of the son disappearing and then having to deal with rumours on top of that.
You need to ask yourself what the core of your "study" is. What can you remove and change and still retain that aspect that attracted you and made you want to dig, fictionally, for more. Is the son or the mother your main focus? Can you change the gender and ages of either, or both, and still retain your story? Do the family have to be foreign? What changes if you make them local/long term residents to the area?
The identifying features of this news event sound like the hyena involvement and the specifics of the rumours/accusations. I would think that lions would be a safer way to go, as hyenas are just a little too specific and connected to that story in particular. There are probably many stories of lions/other wild animals killing people, and it might help you to come up with more fictionalising ideas if you researched some of those news reports and try to blend some different aspects from many stories - so that your theme is about a general occurence: ie "losing a child to wild animals", rather than a specific single event.
Secondly, have you changed the specifics of the rumours? If the rumour was that the parents killed the child, then something like that is also likely to be remembered as specific to this news report in particular, along with the hyenas. If you can, try to take the family out of the story almost completely. Make the suspect a stranger whom the son/daughter meets for the first time in the book and either falls in love with, or develops a strong friendship with, or a substitute mother/child relationship, or even the biological mother he'd never met until now, etc. This new fictional character can bare the brunt of the accusations, etc, and behavioural traits that may make a real person feel like their reputation has been defamed. You could maybe make this character the POV character to show how deeply the relationship developed and how hard the death is on this person (ie, take your mother-character and make her not related to the son-character, or, in the case of the biological mother, not the person who raised the son).
Give the family only scenes that show them as having the reactions any normal person would expect (grief, worry, anxiety), plus extra dignity, honour, and respect for even the person they believe responsible. So if, for example, in real life, a particular family member was reported screaming at the suspect when they were granted bail, change that to show that person being in complete control, although upset. Unfortunately, if you try to keep one of the family as your POV character, they could feel rather flat if you're trying to portray them in an ideal light - so they would really only work as minor characters.
Thanks for posting this example. It really helps us all learn more about our craft.
(PS, I am not a lawyer and this is not intended as legal advice.)