What is best to use on a baby for mosquito repellent that is safe?

Try not to use. Most insect repellents have the ingredient deet which when absorbed through the skin can be neuro-toxic ( hurt our nervous system. Low dose 7-10% deet can be used on children but for babies i would try to use netting. You can spray some on their clothes and stroller as this may help. There are also some organic products which do not contain deet.