This is a difficult topic. We opted for an old straightforward non-smart phone for our 10 year old who commutes, as we didn't think he needed internet access - just to be able to text and call if necessary on the way to and from school. A bonus is that he's unlikely to be the target of anyone wanting to snatch his phone (there have been muggings of kids for their phones - e.g they used it on a bus and had it taken from them when they got off the bus. A rule of not using it while walking down the street is a good one if you do go for a smartphone).
The suggested minimum age for what's app is 16 and I know of several schools where it has been 'banned' for year 6 children as it was used for 'bullying' type behaviour/unkind messages. I hasten to add that the objected to language and messages seemed more a feature of children not thinking through the impact of messages rather than any sort of deliberate bullying (you could argue that at the age of 10 their brains are simply not developed enough to appreciate the tone, context and nuance of this kind of group messaging. I think it can be difficult enough for adults to always get it right, let alone children).
This woman is a useful and non judgemental digital media 'expert' - her blogs are good I think. For what it's worth we will try and hold off as long as possible on a smartphone and certainly on social media and what's app.
Good luck navigating this tricky field! https://drkristygoodwin.com/when-is-the ... martphone/