Children start attending Hogwarts when they are eleven years old, but they come to the school already knowing how to read and write, so they had to learn it somewhere.
And in the wizarding world, there are families where both parents work, just like in the Muggle one, so not everyone just has they're mother teach them how to read. I have read stories about families who hire tutors, and even some where small one-room style school houses exist just to teach children the same basic lessons that children in Muggle primary schools learn.
What are some other possibilities that yo have all read? I would love to hear peoples thoughts and opinions.