@Karretch To be more precise: The phenomenon is called "Childhood amnesia" and people usually don't recall
anything before age four and way (compared to usual memory loss due to passing of time) less before the age of ten[1]. The latter age highly depends on a child's education (especially music or other arts, as it increases the connection between the two parts of the brain) [2] and children from less wealthy families usually develop slower in general. Currently studies are performed in India to see how poverty affects child development, physically and psychologically. The preliminary results are pretty bad. It is estimated that regular malnourishment can throw children back as much as six years at the age of twelve!
In addition to that, children need about until the age of eight to develop an instinct of self-preservation. Yes, they sense danger (via the behaviour of their surrounding, or the lack of the latter) and feel pain, the two key factors and make themselves noticeable, but this is copied behaviour from adults and children can't combine the extrinsic factors into the intrinsic conclusion: "This situation is dangerous and I need to flee or fight". They will do what their parents tell them to do.
A more modern example: Children will (unless instructed otherwise) follow any stranger they don't see as a threat and who baits them into following (candy, telling that they were sent by the parents, etc.) and as behaving non threatening is trivial for other humans, this is a behaviour child molesters exploit regularly.
So to witness an execution and draw the correct conclusions from it puts her in the range of six to ten years, if not older.
[1] Eacott, Madeline J. (April 1999). "Memory for the Events of Early Childhood". Current Directions in Psychological Science. 8 (2): 46–48.
[2] Hayne, H (2004). "Infant memory development: Implications for childhood amnesia". Developmental Review. 24: 33–73.