Thanks for your feedback on the birthday party issue! I decided to bite the bullet and do Chuck E. Cheese--Sabrina would be happy, I could invite her family, etc.
Then I went to court.
Dad's ICPC petition was denied. So dad isn't getting custody.
But Sabrina can stay with him for the rest of the summer.
So I'm off the hook for planning a birthday party at all.
Sabrina will get some therapy to prepare her for coming back to me when school starts, so it isn't a horrible shock for her (though it will still be horrible for her!), and dad is considering moving into our jurisdiction so he can get custody, and the agency will look at dad's sister as a possible placement.
And mom is just crazy.*
*The GAL's assessment, not mine!