Yes prebiotic is good for your baby, I gave my son probiotic+vitaminD daily.

1.Heard can boost the immune system of baby. Any side effect feeding her probiotic?———— yes it’s boost the immune system, no side effects as long as you follow the correct dose and get legit / trusted product. Mine never sick,fever,cough,rashes etc etc since the day he’s born. He’s more than 1 YO now.

2. Will her body get used to it then become not effective anymore? ————— you are so funny, it’s not a medication. No such things are not effective anymore.

Oh you can try BioGaia ProTectis and culturelle. it’s include vitamin D.