My ex-wife's petty behavior has caused our son emotional distress. What can I do about it?

Question: My 33-year-old ex-wife is refusing to share a lunchbox that our 3-year-old son has been using to go to preschool with daily. I know it sounds strange. It is. Let me explain.

I bought him his backpack and she bought him a lunchbox, which we've been switching back and forth on our parenting days for quite some time now. Recently, she blew up about a sticker that was no longer on the lunchbox and sent me a nasty email telling me to purchase my own lunchbox. Now, she refuses to give it to me on the days when I am taking our son to school.

My son's teacher reached out, saying he was in emotional distress during snack time over not having this lunchbox and his normal snacks, yet my ex refuses to let me have it to send with him to school knowing this.