Their ‘universal’ grace covers infants via parents’ possible salvation
Their universal grace covers infants too, not adults only.
God tied infant salvation-power to the parents’ condition.
He made every parent’s own salvation possible.
So children — as parents’ appendages — share that possible salvation.
Latin
XXIV. Gratiam autem illam, quam vocant universalem, docent universalem esse, non tantum respectu adultorum, sed etiam respectu ipsorum infantium, quibus, scilicet, Deus potestatem salutis fecit, quatenus voluit ut infantes sequantur conditionem parentum: omnibus vero parentibus salutem propriam possibilem reddidit: adeoque salutem infantium qui sunt ex ipsis oriundi, & qui habentur tanquam eorum quaedam appendices.