Answer
Annotate the child Application resources with sync waves so the parent applies them in order (e.g., infra/CRDs before workloads). Since the parent sync applies the Application CRs, waves on those CRs sequence when each child is created and begins syncing, letting prerequisites come up first.
# child Application manifest
metadata:
annotations:
argocd.argoproj.io/sync-wave: "0" # infra first, apps laterReal-world example The ingress-controller child app (wave 0) is created before the services (wave 1) that depend on it.