Is it safe ?

You need to use kid safe essential oils. There are some you aren’t supposed to use but some companies like Young Living and Plant Therapy make kid safe oils. Join some Facebook groups and find a reputable aromatherapy website. There’s also a book called Gentle Babies by Debra Rayburn that lets you know which oils are safe for pregnancy, babies, and children.

I think the recommendation is to wait until 6 months.

Not a good idea.