Update: FIL gave it to DS on Xmas Eve, and told DAD she would get one when she is 10.
We joked for the next few days about reminding Grandpa not to forget when she turned 10. If he does forget, we will buy it.
She was a tiny bit sad when he opened it but overall was fine with it.
Late to the fray (dealing with own family holiday stuff!), but FWIW, I think this is a good lesson -- specifically, that "equal" does not mean "the same," and that our love for them is not measured by the size/cost of their gifts.* And I think those lessons are far more important in the end, even if the short-term cost is some hurt feelings.
My kids are 4.5 years apart, and so we have already had several occasions in which the elder got something significantly larger than the younger, usually for birthday or Christmas (because, come on, if I'm going to pay for something like a damn laptop, I'm going to get credit for it as a present!). They have been very clearly given and received as "age-specific" gifts -- the laptop when it was needed for school, the phone when DD was first coming home alone, etc. And so now DS is making his own mental list of when
he gets those things. :-)
But there are also times when one or the other gets more -- for ex., we have at different times gotten the kids sports stuff related to the sports they were interested in, when we didn't have something comparable for the other kid. For those, the kids can be a little disappointed, but they just have to learn that it evens out over time, and that the relative value of their gifts does not reflect their relative value to us.
*I am particularly sensitive to this, because I still remember visiting my dad and his new family at Christmas and seeing a huge giant pile of wrapping paper from all the stuff his boys opened, and the two little gifts they had saved for me. No one ever tried to make it "even," or to make sure I knew that he loved me as much as he loved them. It wasn't until I was in my 30s and learned some of the backstory that I finally learned that lesson -- and I still have a weakness for big gifts. So it is a big deal for me that my kids actually get to experience that little bit of disappointment, but in an environment where they know for sure that they are loved and valued, so they don't learn that "love" = "stuff."