Is it illegal?

Yes. According to Section 146 of the Crimes Ordinance of Hong Kong,
a person commits a criminal offence if he / she commits an act of gross
indecency with or towards a child under the age of 16, or who incites a
child under the age of 16 to commit such an act with or towards him or her
or another, and shall be liable on conviction on indictment to
imprisonment for 10 years.

2010-01-13 21:58:13 補充：
刑事罪行條例SECT 146 - 向年齡在16歲以下的兒童作出猥褻行為:
任何人與或向一名年齡在16歲以下的兒童作出嚴重猥褻作為，或煽惑一名年齡在16歲以下的兒童與他或她、向他或她、與另一人或向另一人作出此種作為，即屬犯罪. 一經循公訴程序定罪，可處監禁10年。

係違法, 但都係屬於非禮罪哩.