I think the greatest difficulties experienced would be social. Not that they wouldn't develop social skills, but they would feel and likely be looked upon differently by their peers. They could possibly feel disconnected and may even feel like an outcast. People are generally judgemental, especially around certain ages. This isn't necessarily a reason to give a child a smartphone (quite the opposite) but it is a point of discussion.
It's probably not practical to completely isolate them from the internet. Today's education system relies heavily on computing, so any student will encounter internet-connected devices sooner or later. It's best in my opinion to introduce it to a child as a tool that can be used for both productivity and recreation. Making it a 'forbidden' thing will only increase the appeal. The key is balance. While they should know how to use computers to accomplish necessary tasks, it should not be the focus of their existence. Balance should not be first mentioned within the context of computers, but rather taught as a central tenet of the human experience. Someone who already understands the importance of balance will be able to use computers responsibly. With that said, people will do what they want to do lol.
I can't speak to all 5-6 year olds, but I will say that the ages at which children are using or being given devices of their own is decreasing. Until it's an absolute necessity I'd recommend waiting on a smartphone. That kind of connectivity/accessibility has proven to be a double-edged sword for adults, let alone children. They should have access (controlled by an adult) to a wifi only device such as a laptop that can be used at home. You can prevent them from installing software, allow/block certain domains, and so on if you have basic working knowledge of computers.