I would highly recommend a boxer. My uncle has a boxer with small kids around and she is VERY protective of the kids. They have never had any problems with the boxer and the kids. Very very good dogs!! Jack russels are good dogs to but I have always been warned about them because they can turn and I have always been told they have a locking jaw so watch out. Good luck with your search for a dog.
First question, has your Aun every owned a dog before? Puppies are sooo cute but do require a lot of attention. Since your aunt has two small children (who need alot of attention as well) why not consider a little older dog. You and she could check out a shelter in your area. At first I would suggest NOT taking the kids(voice of experience, if they go along you are coming home with a dog) but if you should find one ask if they would hold it a few hours or one day so you could bring the kids down and see how they react to the dog and the dog reacts to them. I am talking like a l., 2,3 yr. old dog. At those ages you would know what you were getting, size wise, temperment wise and coat wise. We went to our Humane Society and got a wonderful pet, we had him for 13 years, after we had to put him to sleep we needed to be sure we got a small dog and our Humane Society didn't have any but we found an ad in the paper and went to "just LOOK" :) at a Bichon Frise/Westie mix, he came from a small litter, only 4 males, it took my husband, my mom, and myself 2 hours to pick Cody out.He is a great dog and he is a year old now. Good luck.