What kind of dog sounds right for my family?

first my mom wants a working dog we already have a year old english mastiff she loves other dogs and has to be good with kids because my cousin visits alot he is 7 I have alot of time to excersie the dog because I am home schooled and my mom is at home and we also want a dog that isint prone to alot of health problemsand is freindly and protective also intellingent thanks for all awsers!