Hm, I always try to think best of other people unless proven otherwise. So maybe for her it was just a lucky coincidence that her other son's birthday was right around the time of the photo shoot you donated the cookies for? She used them for the photo shoot and instead of throwing them away afterwards, she gave them to her son for his b-day.
As long as this professional photo shoot did actually happen and you will receive the promised pictures, why not go down that road? Though I admit I would watch her a bit closer after that and check what becomes of my donated cookies