All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
As for the granddaughter taking what her grandma gave her the permission to take, then there is no blame on her at all if she does so; since it belongs to the grandma and she has the full right to dispose of it as she wills, as stated in the question.
As for performing Sujood As-Sahw (prostration of forgetfulness) after saying Tasleem and the time limit of that, then this can be known by custom, even though some Muslim scholars view that it is permissible to do it a long time after saying Tasleem. Shaykh Khaleel (from the Maaliki School of jurisprudence) said in his Mukhtasar what means: "One should prostrate after Tasleem even after a month has elapsed."
Kindly, refer to Fataawa 83932, 85480 and 90941, for they have an explanation about some rulings of the prostration of forgetfulness and that its time limit can be fixed in compliance with the known custom.
Allaah Knows best.