Living with a child who wont sleep

I have this beautiful Grand daughter named Bella. She is a hoot and such a joy until she decides to refuse to sleep all night. She is three years old and I have no idea why she is like this. One night she sleeps and then she gets on a roll and wakes up at anywhere between 2am to 4 am sometimes multiple times like last night. Any ideas what I can do to keep her asleep outside of medication or using lavender.