Q. Can Small Business Server (SBS) be a child domain in an existing Active Directory (AD) forest?

A. No. Like Enterprise Business Server, the SBS domain must be the AD forest root domain, and it can't have any child domains. The SBS server must also hold all the domain Flexible Single-master Operation (FSMO) roles, and no trusts can exist with other domains.

See the Microsoft "Requirements for License Compliance" Web page for a licensing restrictions list.