Diffuser in LOs room

Looking for some advice. I have been thinking about getting a diffuser for my 21 month olds room. She is obsessed with smelling my body wash that is lavender scented so I thought she might like her room smelling like that and bonus if it helps her sleep. I have never used a diffuser. I don’t plan to have it run all night. Probably just few a couple hours before bedtime. Any recommendations on amount of essential oil to use? I am a little confused on that. Is there a good brand besides young essential? That brand is pretty pricey but seems highly rated. Thanks for any advice.