How can I stop my dogs from barking?

I have 2 dachshunds and every time we go camping, they bark at the people ping by, or they bark at little kids on bicycles going by. They aren't really friendly, people ask if they can pet them, and we tell them they cant, because they bark at them and act very aggressive. Its not only when we go camping, they bark at the people going by at home. I just dont know what to do anymore, we have used bark collars and that hasnt worked. Please help me.