Merge branch 'FATRAS_childrenToParticleBroker_21.3' into '21.3'

Manual sweep of !9379 into 21.3: Making sure Fatras pushes children in the particle broker

See merge request atlas/athena!9578

Former-commit-id: 8b02b2d5565ded3a4f164fdad643230490c55d8a