Alice de Quincy did not remarry and did not have the 3 Thornhill children. Especially the 2 youngest when she was 55.
Richard de Thornhill married Alice de St Liz (1150-1204) in 1165, daughter of Simon St Liz, earl of Huntingdon and Northampton.
Here is Simon's page: https://en.wikipedia.org/wiki/Simon_II_de_Senlis,_Earl_of_Huntingdo....
His Geni page incorrectly lists his daughter as "Amice" instead of Alice which could be the source of the confusion.
Here is Alice Senlis (St Liz)'s father's page: Simon de Senliz, II, 4th Earl of Huntingdon & Northampton