Is it ok to give Melatonin to a 18 month old baby to help the baby sleep?

Although Melatonin can be effective in adults, it is not a good idea to give your baby supplements without consulting the pediatrician. If your 7-month-old is having enough trouble sleeping that you are considering medicating him/her, than there very well could be an underlying problem that needs to be addressed. Self-medicating is very likely not the right answer to your problem.