Two thoughts on this one;
First I am wondering if Charlotte was a second wife to Thomas, as there is quite a gap in th children and she would have been so young to be mother to Juliana. no evidence as such, just a thought.
Secondly, out of interest, there are marriages for some of the children on FreeBMD if you have not already found tham (I am sure you have)
Ap/May/Jun 1845
Juliana Selby Bettle
Newport Pagnell
Apr/May/JUn 1854
Thomas Selby Bettle
Daventry
(this scould be the son,or Thomas senior if charlotte died pre 1854 - though you have them on 1861, so must be son)
Oct/Nov/Dec 1850
Margaret Selby Bettle
Newport pagnell
(not on your list above, but looks like another child!)
Ap/MAy/JUn 1861
Elizabeth Selby Bettle
Newport pagnell
A/M/J 1862
Emma Selby Bettle
N'port P'nell
also a Frederick S-B, a Jonathan S-B and a William S-b married elsewhere (other than NP)