To reproduce:
- Activate analytic accounting
- Create an analytic plan
- Click on Subplans
- Create a Subplan
- Come back to the list view of subplans (with the breadcrumb)
=> The new plan does not appear

The children_ids are not recomputed, so we inverse the logic.

task-3359643

closes odoo/odoo#124404

Signed-off-by: William André (wan) <wan@odoo.com>