Here's what I know now.

After I went to bed last night both boys got up and were messing around from 11PM to 3AM.  I heard noises a couple of times and got up to investigate, but they did a good job of pretending and hiding.

This morning Middle Boy told me he was the only one up and that it was Sophie making all of the noise during the night.  He had allegedly gone to bed at midnight.

Younger Boy then told me that Middle Boy "forced" him to get up and that they were up until 1AM.

I have decided we will sit down after school and talk about what happened and who did what.

Prayers for wisdom for me during this discussion.  I am so frustrated by the two of them and their bedtime/nighttime antics right now.