John Leslie is on the '51 census - aged 2 living with his grandparents James and Elizabeth Leslie at 47 Gellymill Street Macduff.
That does explain why I didn't find him - I was looking for a one-year-old born in Deskford with parents John and Isabella, not a two-year-old born in Gamrie with parents James and Elizabeth.
The probable reason that you can't find him is that on the transcripted records that we can access for the Scottish censuses the family name is given as 'Sesler'!
I don't know which transcription you are using, but you have the same access as I do to FreeCEN and to Scotland's People. John is correctly indexed on both of those.
The transcribing of census records by the powers that be in Scotland is a nightmare!
As stated above, John is correctly indexed on Scotland's People, which is the
only source with which the 'powers that be in Scotland' have anything to do. This is also the
only place where you can view, instantly, online digitised images of the original census documents to check the accuracy of whatever transcription you have looked at.
You must not try to blame 'the powers that be in Scotland' for the inaccuracy of transcriptions made by other organisations, some of which are notorious for the .... er .... let us say creativity of spelling in their transcriptions
FreeCEN transcriptions are done by volunteers who know the areas they are transcribing. Every transcription is done twice and if the two do not coincide, the co-ordinator and transcribers will review the result before it is published.
If John was married or died in Scotland, his marriage and death certificates, available on Scotland's People at modest cost, ought to tell you the full names of both parents, including his mother's maiden surname.
I note that the SP baptism index has no likely candidate for John. It may be that he was illegitimate. If so, it is quite likely that the kirk session minutes of either Deskford or Gamrie might mention his parents. These are held in the National
Archives Records of Scotland. They have been digitised but it has not yet been possible to put them online, so you would need to hire a searcher if you cannot travel to Edinburgh/Glasgow/Aberdeen etc yourself.