It is lovely isn't it, to hear someone regard their DC as their best friend(s?)
Mine are around 30 now, and I do have a wonderful relationship with them, and they DO want to spend time with me (and DH.) I am sure yours will be the same. 
I don't see anything wrong with saying your child(ren) is your best friend. I guess you get annoyed when someone says their husband is their best friend too hey @ThatArtfulCoralFinch ? 🙄 I don't know why it bothers you, but yeah my 2 DD are 2 of my best friends, along with DH. I also have 2 other close friends (one from school and one in my village.)
Definition(s) of 'friend.'
Someone who accepts you unconditionally, helps you grow, supports you in tough times, and increases your ability to love yourself.
A person with whom one has a bond of mutual affection.
Someone you trust and with whom you share a deep level of understanding and communication. A good friend will: Show a genuine interest in what's going on in your life, what you have to say, and how you think and feel. Accept you for who you are.
THIS describes my 2 adult DC and my DH. You don't have to be unrelated to someone to be a 'friend.'
You sound a bit judgmental and somewhat bitter to be honest.