APM V11.1HF1 querying Active Directory

Right - LDAP auth is not going to work well/easily for multiple domains, as the user needs to be authenticated against each individual AD controller that's responsible for the domain. Do you have Cross Domain Support set to Enabled? Are your users entering their credentials in the full UPN notation: user@child.domain.com? Is APM able to reach all domain controllers responsible for each of the child domains?