I finally did it!

It feels more like a patch than anything else, but it'll do the job.

In the automation rules, I've added a “on registration” rule, for all my custom fields, with the following python code:

enfant = record.env.user.x_enfants.search([]).filtered(lambda e : e["x_name"] == record["x_name"]).filtered(lambda e : e["id"] != record['id'])

savedName = record["x_name"]

record["x_name"] = record["x_studio_parent"]

if enfant:

    enfant.write({

        "x_studio_age_mois" : record["x_studio_age_mois"],

        "x_studio_poids_kg" : record["x_studio_poids_kg"],

        "x_studio_taille_cm" : record["x_studio_taille_cm"],

        "x_studio_commande" : record["x_studio_commande"],

        "x_studio_parent" : enfant["x_studio_parent"],

        "x_studio_notes" : record["x_studio_notes"]})

else:

    record["x_name"] = savedName

In this code, we retrieve the "enfant" we're interested in with a filter. Then we edit its content with the record data.

The savedName thing is to avoid creating a new object with the x_name currently being parsed, which would belong to the current user. The “else” case is to continue managing the creation of an object in the normal way .

The only problem is that it creates objects to be destroyed afterwards that belong to the OdooBot user, so I think another automation rule to clean this up will do the trick.