Far be it from me to assume I know how your daughter should react. But I do have a marvelous idea. This would be a great lesson in forgiveness.
First you sit down with your daughter and explain to her what happened.
Tell her the neighbor couldn't know how much the flower meant to her.
Explain to her what Jesus said about Forgiveness.
Then Peter came to Jesus and asked, "Lord, how many times shall I forgive my brother when he sins against me? Up to seven times?"
Jesus answered, "I tell you, not seven times, but seventy times seven."
Then lovingly define forgiveness for her. "Honey, forgiveness is when you give up your right to hurt someone for hurting you."
Then tell her she should confront the neighbor after a little time has past and tell him to his face that she forgives him and she wants to know if he would like to help plant another sunflower next year or the next possible time.
Now of course it will never go off without a hitch. But that doesn't mean your daughter can't learn a great lesson on how to forgive even though she might not want to forgive.
Lastly, I want to remind you that you solicited advise and that nothing I said here persumes to make any judgment on you or your family. For all I know this was the first thing you did. Nevertheless, may God Bless Your Daughter, your family and the neighbor child. I hope everything turns out great.
Thanks, this is helping!