Child AD upgrade from 2008R2 to 2012

Hi All

Our production envriorment Parent forest function level is 2003 and the child domain funcation level is 2008 R2, Now
I want to upgrade our child domain level to 2012 (keep Parent forest level 2003), Is there any problem when I doing this upgrade(domain replication issue or something else)?
Or could someone show me some any best practice from Microsoft？