There's a really good post here by Davide Mauri explaining the order in which "things" happen when a package spins up. By "things" I mean logging starting, reading configurations, validation and control-flow execution.
Logging With SSIS: Beware of Parent Package Variable configuration!
The important point to take away from this is that parent package configurations are applied at a different point in time from all other configurations. That's an important thing to remember.
Its well worth 30 seconds of your time to read.