As quick note for my self. To install new (without co-exist legacy) Exchange in child domain, below is the step :
Scenario : Parent-Child domain (parent.local & child.parent.local)
1) In parent.local domain controller, sign in as account have Enterprise and Schema Admin for parent.local.
2) Open command prompt and perform : setup.com /PrepareSchema
3) Once completed, in same login perform : setup.com /PrepareAD /OrganizationName:parent.local
4) After completed, in child.parent.local domain controller, sign in as account have Domain Admins for child.parent.local.
5) Open command prompt and perform : setup.com /PrepareDomain:child.parent.local
Finish and can continue to setup Exchange 2010.