You already have a very good explanation. The only thing I'd add is that Aunt Alexandra clearly feels responsible for the children in the way a mother would. She has undergone a change since she's been staying with them. When she first came, Aunt Alexandra was prepared to "fix" Jem and her brother--and Scout, in particular. Over the course of the summer, she has softened and is beginning to understand why Atticus is doing what he's doing and that the kids are actually pretty well adjusted--despite their disregard for their heritage and manners and despite the influence of Calpurnia. She has grown to love them. So, she not only has the feeling of foreboding mentioned above, she also has a mother's instinct for these kids, the idea that she may have been able to help if only she'd been there.
In this part of the story Scout and Jem have just been attacked on their way home from the pageant at school. Aunt Alexandra is, of course, beside herself. Neither she nor Atticus accompanied the children to the play and likely this guilt comes as a result of the maternal side of Aunt Alexandra that tells her she should have either gone with the children to the play, or made them stay home.
Don't forget that just a few pages before (in Chapter 27) when Scout is giving her mini-performance for Atticus, Alexandra and Cal, Aunt Alexandra has a chill and says, "Something just walked over my grave." This was a moment of foreshadowing that something bad was going to happen later - and when it does - no doubt Alexandra thinks she is partly responsible because she did nothing back when she had the foreboding feeling.
In reality, however, realize that the events of the evening are not in fact Alexandra's fault. She likely could have done nothing to stop them even if she had been with the children.