The section that most resembles your description seems to be a portion of chapter XXXV and XXXVI, where Renzo finds Fra Cristoforo in the lazzeretto. There you can find a few descriptions of him and other Capuchin frias while they tend to the sick. For example:
> Arrivò in quel punto un cappuccino con la barba bianchissima, portando due bambini strillanti, uno per braccio, raccolti allora vicino alle madri spirate; e una donna corse a riceverli, e andava guardando tra la brigata e nel gregge, per trovar subito chi tenesse lor luogo di madre.
* * *
> At that moment a Capuchin friar with a very white beard arrived, carrying two screaming babies, one on each arm, picked up just at that moment from the deceased mothers; and a woman ran to receive them, and she went looking through the group and the crowd, to find immediately who could play the role of the mother for them.
>
> _(my translation)_