I agree about putting it back onto the mom.
I'd talk to her today in advance.
"Susan as you know Sibling has shown an interest in having his own share of what I give to my daycare children. Now at first I was fine with it but Im afraid of it continuing and becoming a regular thing. I don't want to hurt his feelings which is why I wanted to talk to you about it. Tomorrow ill be giving the daycare kids baskets for Halloween and I have enough to make twelve. Like I said, I dont want to hurt Siblings feelings. Each basket will cost me $7 to make so if you'd like him to have one tomorrow you're more than welcome to leave me the $7 today and ill get supplies to make an extra one for him also"
For the rest of the time you simply have to say no when he asks for things.
"I'm sorry Sibling. I only made enough witches hats for each of my daycare kids and I don't have an extra one. I'm sure mommy will make one for you at home of you ask her. "
"I'd love to give you an apple Sibling but I need those apples for tomorrow and I only have enough for my daycare kids. I'm sure mommy will give you an apple when you get home. What? You don't have apples at home? I'm sure mommy will get some when she goes to the store. "
One thing I do is to not leave any supplies out including food. All of my supplies and food is put away and I only pull out what I need for each child. If a sibling or parent as me for one "to go" I just say I don't have one to give.