curl https://api.tracklysms.com/api/v2/accounts/children/1042 \
  -H "X-Api-Key: trk_your_parent_key"

{
  "child": {
    "accountId": 1042,
    "name": "Acme West",
    "status": "active",
    "plan": "paid",
    "externalIds": { "partnerId": "p_1", "businessId": "b_9", "locationId": "loc_west" },
    "billedByParent": true,
    "createdAt": "2026-07-26T14:03:11.123456"
  }
}

{ "error": "Child account not found", "code": "child_not_found" }