Yes, that's right. The notion of a reasonable person is not wholly objective. Just as the notional 'reasonable person' is taken to have a particular defendant's knowledge, capacity for care and foresight, so inescapable attributes such as youth are relevant in deciding what ought to have been foreseeable in a particular case.
Thus, A would not be expected to foresee the risk of harm that may have been foreseeable only to an adult in the circumstances. She would only be expected to meet a standard that is normal for children of the same age and general experience as herself.
Nevertheless, in this case, it is strongly arguable that even on this test, the risk of harm likely to result from playing with a small child and a ball near a street would have been foreseeable to an average twelve-year-old child.