Some people just didn't bother registering their kids, even though there may have been penalties.
Some people were missed off the indexing, although if you really can't find anyone, it might be worth a visit to your local archives to look through the GRO fiches.
Some births, deaths and marriages might not have made it to the National Register Office, although you might find them in the local Register Office indexes.
Some people were missed off the census because they were on holiday, didn't fill in the form or weren't in with the census enumerator called, or of course they didn't want to be counted.
Although registration of births etc was introduced in 1837 -- it was not made compulsory until about 1872 (I think that was the date) so certainly some people didn't choose to register there children before that