A trip to the library for some books about how to raise your new puppy will be very helpful. If you can find one called Good Dogs Great Owners by Brian Kilcommons that is a good one. With two small children it is critical that you teach your children how to relate to a young puppy as well as teach your puppy how to live with your family.
Whether or not your dog turns out to be a wonderful addition to your family or "mean" will depend mostly on how you treat it. Congratulations on adding a dog to your children's lives. Let us know how things go.