47 years ago, I had a neighbor couple who were in their 70's who allowed me to tag along into their gardens. I was 4 then. All they "did" was talk about why they loved this or that plant, that THIS plant was given to them by their daughter on Mothers day when she was in the 3rd grade, that plant has been in this garden since long before you were born- things such as that. They passed on their love of gardening- never realizing that it would begin a career in horticulture that has served me for the rest of my life.
Funny, I am now doing this with my nearly 4 year old Son and it is a true delight. After 7 non-interested kids before him, I have hit pay-dirt!
What I might recommend is to add the human element into all of it. Plants and people have a lot in common- these are things she will understand. We just "do" our things differently. My Son understands that when it rains, the plants are drinking! When the sun is shining, the plants are eating! When the leaves fall off, they are going to sleep. Go online and find one of those videos that show plant movement speeded up- she will be amazed! They move like us- just slower! Once I was planting seeds and I told him I was putting them in their crib. They would wake up in a week. He remembered that and now he will tell me it is time to put the seeds in their cribs! lol.
Mainly, do not worry to much- your passion will be all she needs.