I chose usufruct in my late husband’s estate. His children (bare owners) now want to convert my usufruct in the main residence into capital. Can they force me?

No. The law allows conversion of usufruct into money in the form of a life annuity (Civil Code, Article 759). Without agreement, the judge decides (Article 760).
However, if the usufruct concerns the surviving spouse’s main residence, the judge cannot order conversion against the spouse’s will (Article 760).