my "girlz" do not like small children

Hi, my girlz are 4 and 5 yrs old and didn’t grow up around small children. They do not like small children-they get very nervous and one of my girlz nipped a 7 year old boy (last summer) who tried to pet her. My oldest daughter is going to have a baby in 6 months, and I will be babysitting a few days a week. I’m very afraid of how my girlz will be towards the baby. My girlz are very protective of me, especially my blue merle Jazzy…my kids have told me that she (Jazzy) is fine (to their friends) when I’m NOT home, but when I’m home she is very protective and amped up when their friends come over..ONLY when they first arrive, after they’ve been here a few min. or so, Jazzy is fine with them! I’m starting to think that Jazzy was just born to be a great protector and it’s her personality trait..something that quite possibly can’t be changed/fixed!? I’ve spent close to $1000.00 on two different trainers..not much help. My biggest concern is how to get my girlz (mostly Jazzy) to NOT be aggressive toward the baby. I would GREATLY appreciate any suggestion!!! Thanks!