Based on what you've said I strongly suggest you think about what you will do if the middle person doesn't want the window seat -- or perhaps doesn't want to move at all. Are you prepared to have a stranger sit in the middle between you and your granddaughter? If so, then OK. If not then book aisle and middle to begin with.

I know that's not what you want to hear but it's really the only way to be fair and avoid an uncomfortable situation.