For the movies sake i would go with Fredo. Kay may have changed her clothes in the bathroom and was too tired (it was a long day being in the party) to notice the drapes had been left open and when micheal wakes her up the whole thing sets in motion.

I also think Fredo didnt know it was going to be hit (somebody said something about a kidnapping, sounds plausible as fredo was stupid enough to think it would be harmless).

Even if one of the kids left the drapes open, for whatever reason Kay was too tired to notice.