Try this U forget to set parent reference in the child (In the for loop mb.setParent(lbl);)
for (int i = 0; i < list.size(); i++) {
System.out.println("inside library model edit");
mb = new LibraryBarCodeEntity();
System.out.println("bar[i]" + bar[i]);
System.out.println("bar[i]" + accission[i]);
mb.setBarCode(bar[i]);
mb.setAccessno(accission[i]);
para.add(new Phrase(new Chunk(BarcodeDao.genarateBarcode(bar[i], writer), 0, 0, true)));
para.add(new Phrase(Chunk.NEWLINE));
para.add(new Phrase(Chunk.NEWLINE));
mb.setParent(lbl);
s.add(mb);
}