Agnes de Arches and Herbert de St Quintin had at least three children Alice, Walter and Alan. Walter and Alan predeceased their mother as shown by a gift dated 1144-55 to the nuns at Nunkeeling, which establishment she founded, "pro salute anime domini mei Herberti de Sancto Quintino et pro animabus Walteri et Alani filiorum meorum." [Farrer. EYC v.3 p.53]. She does not appear to be mother of Richard de St Quintin, Herbert's son and heir.
Agnes is referred to Alice's mother in a gift made to Nunkeeling by the latter between 1164-1170. Alice married Robert son of Fulk the Steward and secondly Eustace de Merc. She appears to have at least four children by her first husband William, Walter, Henry, and Sybil.
By her second marriage to Robert de Fauconberg, she was mother of Piers/Peter, his son and heir; and by her marriage to William Foliot she was mother of William and Hugh Foliot. This is shown by the confirmation by William, archbishop of York, dated 1143-1154, of a gift made by her to Nunkeeling by "petitione Agnetis matris Petri de Falcamberga et filiorum suorum Willelmi et Hugonis Foliot" [Farrer.EYC v.3 p.54]