Is it safe to give buclizine hcl, an appetite stimulant, to a 13months baby? She's 8kg. How much should I give?

NO! Do not give this to your child. It is not meant to be given for that reason. It is not the right way to get your child to gain weight. You need to talk to your child's doctor about any appetite/weight gain concerns. Medicine is not the answer, and that medicine could cause serious harm to your child.