Oddly enough, Drew (almost 11) didn't ask any questions about it until he saw my new avatar (thank you, Dawn!). I explained to him that it was to show solidarity with England over the recent bombings there. Since he hadn't heard about it, I told him and explained their relation to 9/11 and the attacks here. His comment was he hopes they "catch those guys and they fry for it." Not exactly the comment I had hoped for, but I have to admit I agree with him.
Ally hasn't asked about it and I don't plan on telling her anything unless she does. She's only 4 and I don't think anything more than saying some bad people did some bad things is necessary. Quite honestly, I don't think she would really understand it.