Anakin knew that it was most likely a little strange for the young girl. Some man she didn't know offering to be her guardian. He wasn't exactly sure what compelled him to offer to be the girl's guardian. Perhaps it was the fact that she looked like Padme. A young version of Padme, yes, but Padme nonetheless. And he knew that she needed someone to care for her, someone to watch over her.
So there he was, meeting Mathilda for milkshakes. He wasn't sure he'd actually ever had a milkshake, he just knew that Hinkles sold them. So he had her meet him there.
She apparently had gotten there before him, because he spotted her at one of the booths inside. "She's right," he told the waitress as he approached. "I'll pay."