All right. I kept tweaking the message by trial-and-error, and running it through Google translate, until I got something that still made sense and sounded respectful on its way back to English (I assume that therefore it will be likewise in the Japanese).
Here's what the message now says:
It appears that your daughter visited the blog of a celebrity named Gulliver, and she discovered this website-- our fansite for him-- from a link one of us had left in the comments on his blog. She casually mentioned that she met him, so I sent her a message through this website because as a huge fan I wanted to know more. I didn't know that my message would worry you.
Please accept my explanation, my profuse apologies, and my good wishes.
Here's the Japanese translation:
私たちのひとりが彼のブログにコメントに残されていたリンクから - 彼のために私たちのファンサイト - それはあなたの娘はガリバーという有名人のブログを訪問し、彼女がこのウェブサイトを発見したことが表示されます。彼女は何気なく彼女が彼に会ったので、巨大なファンとして、私はもっと知りたいと思いましたので、私はこのサイトを通じて、彼女にメッセージを送ったと述べた。私は私のメッセージはあなたを心配することを知りませんでした。
カラ - スアン
And here's the translation back into (broken) English:
Dear Sir or Madam,
Your daughter to visit the blog of celebrity that Gulliver, that she found this website - fan our site for him - from the link that one of us was left in the comments on his blog it will be displayed. I said so because she thought she had met him casually, as a huge fan, and I want to know more, through this site, and I sent a message to her. My message is I did not know that worry you.
My profuse apologies, my explanation, please accept my good wishes.
Kara - Suan
Should I go ahead and send her the PM now, or wait a while?