Article 917 of the French Civil Code allows the reserved heirs, if their reserved share is encumbered by a legacy in usufruct, either to accept it or to request that the disposable portion be allocated to the legatee. This second option creates joint ownership between your partner and your children, which contradicts your intention to protect your partner.
Since this article is not mandatory, you may expressly exclude it in your will, thereby ensuring your wishes are respected while preserving the heirs’ rights, who will receive compensation if their reserved share is infringed.