Petit-Saconnex (note official spelling) is part of the Canton of Geneva, and therefore many of its records are included in the web site of the Archives d'Etat de Genève (AEG). The main database of the AEG, "Adhémar", is at https://ge.ch/arvaegconsult
. Look for the tab "doc. en ligne", then select "Répertoires de l'état civil" from the drop-down menu for indexes covering the period circa 1560-1885 for the entire Canton. In addition to the indexes, also notice the selection "Registres de l'état civil".
Petit-Saconnex was an independent commune within the Canton of Geneva until 1930, when it was absorbed by the city of Geneva.
In addition to the stunning resources on the AEG web site, there is a very large collection of records on microfilm for the Canton of Geneva in the catalogue of the Family History Library on FamilySearch.org. After using the indexes to the "état civil" (this term covers both the church records and the later civil registration system) on the AEG web site, you should follow up by examining the original records on the microfilms, if they are not already in Adhémar.
There is also a web site with the entire run of the Journal de Genève, completely indexed, 1826-1998: http://www.letempsarchives.ch
. There are many other resources, depending on the period that interests you.
Switzerland is really a very small place. Genealogists quickly learn the names of the families. Always include names, places, and dates in your queries! Someone may already have exactly the information you need.