When are you flying? Is it after 1/26/26?
If it is after you will have seats assigned to you if you don't purchase them.
If before that date then it is open seating based on when you check-in with the app/website.
Family boarding will allow up to 2 adults to board with the 6 year old but sometimes they allow more to board together.
Family boarding is done between A and B boarding groups so in everyone in your party gets into the A group it is not needed at that point.