Task: Split nested items with the Split In Batches node

n8n workflow JSON:
{"nodes":[{"name":"On clicking 'execute'","type":"n8n-nodes-base.manualTrigger","position":[250,300],"parameters":{},"typeVersion":1},{"name":"Start","type":"n8n-nodes-base.start","position":[250,300],"parameters":{},"typeVersion":1},{"name":"SplitInBatches","type":"n8n-nodes-base.splitInBatches","position":[650,300],"parameters":{"options":{},"batchSize":1},"typeVersion":1},{"name":"IF","type":"n8n-nodes-base.if","position":[1420,50],"parameters":{"conditions":{"boolean":[{"value1":true,"value2":"={{$node[\"SplitInBatches1\"].context[\"noItemsLeft\"]}}"}]}},"typeVersion":1},{"name":"SplitInBatches1","type":"n8n-nodes-base.splitInBatches","position":[1060,300],"parameters":{"options":{"reset":"={{$node[\"SplitInBatches1\"].context[\"noItemsLeft\"]}}"},"batchSize":1},"typeVersion":1},{"name":"Set1","type":"n8n-nodes-base.set","position":[1280,300],"parameters":{"values":{"string":[{"name":"id","value":"={{$node[\"SplitInBatches1\"].json[\"id\"]}}"}]},"options":{},"keepOnlySet":true},"typeVersion":1},{"name":"Input data","type":"n8n-nodes-base.function","position":[450,300],"parameters":{"functionCode":"const inputData =  [\n  {id: 1, data: [{id: 10}, {id: 11}] },\n  {id: 2, data: [{id: 20}, {id: 21}] },\n  {id: 3, data: [{id: 30}, {id: 31}] },\n];\n\nreturn inputData.map((res) => ({json: res}));"},"typeVersion":1},{"name":"Flatify data with parent id","type":"n8n-nodes-base.function","position":[850,300],"parameters":{"functionCode":"const res = []\nitems.forEach((item) => {\n  item.json.data.forEach((d) => {\n    res.push({...d})\n  })\n})\nreturn res.map((e) => ({json: e}));"},"typeVersion":1},{"name":"IF1","type":"n8n-nodes-base.if","position":[1580,-60],"parameters":{"conditions":{"boolean":[{"value1":true,"value2":"={{$node[\"SplitInBatches\"].context[\"noItemsLeft\"]}}"}]}},"typeVersion":1},{"name":"NoOp","type":"n8n-nodes-base.noOp","position":[1780,-160],"parameters":{},"typeVersion":1}],"connections":{"IF":{"main":[[{"node":"IF1","type":"main","index":0}],[{"node":"SplitInBatches1","type":"main","index":0}]]},"IF1":{"main":[[{"node":"NoOp","type":"main","index":0}],[{"node":"SplitInBatches","type":"main","index":0}]]},"Set1":{"main":[[{"node":"IF","type":"main","index":0}]]},"Input data":{"main":[[{"node":"SplitInBatches","type":"main","index":0}]]},"SplitInBatches":{"main":[[{"node":"Flatify data with parent id","type":"main","index":0}]]},"SplitInBatches1":{"main":[[{"node":"Set1","type":"main","index":0}]]},"On clicking 'execute'":{"main":[[{"node":"Input data","type":"main","index":0}]]},"Flatify data with parent id":{"main":[[{"node":"SplitInBatches1","type":"main","index":0}]]}}}