It IS your place to say something.
The boy, is being a tyrant. Against your son. At pick up time.
This is your son. You have to stand up for him.
He is YOUNGER than that boy, as well.
And it is just NOT proper behavior.
He is STILL in your home.... when this is going on.
I have done home daycare before too.
If another child was being that way to my child, I TOLD the Mom/parents.
And this did happen to my Daughter. The girl I watched was 2 years old and was a real "Alpha" kid... and she really tried to over-rule my daughter.
My daughter would stand up for herself and did not want to share everything either. It is HER home. HER things. I did NOT make or expect, my daughter to Share everything with the other child.
I had toys/things for the other kids, and then clearly made her room OFF limits. I taught the other kids that. And my daughter.
It is our home.
Not everything in it, is up for grabs by the other kids.
The other kids have to learn that too. That is just the rules.
I explain that, to the other kids.
Your son is young. So you do have to stand up for him and speak up... because he is only 18 months old. They cannot defend themselves, verbally or conceptually, by themselves.
The other kid is 2 years old. And compared to your son, that is a real leap developmentally.
Again, when that happened to my child, in my home and I was the care-giver... I DID speak up and talk to the Mom about it.
That is what I am there for.