I don't understand why this isn't ,,Millionen von syrischen Kindern......" or "Millionen syrischer Kinder...." instead of ,,Millionen syrische Kinder...", that is to say, either using dative or a more obvious genitive.
I came across the same thing a while ago when I saw ein Glass Wasser and wondered why there is no von used there. Surprisingly, I realised that in my mother tongue, Persian, we use it in the same way with no preposition:
یک لیوان آب بده لطفا Give me a glass of water please.
And if in Persian we add ِ , the meaning could change in some circumstances:
یک لیوانِ آب بده لطف Give me a glass which I can use for drinking water.
The persian equivalent of "of" shown by a small slanted dash in the second example changes the meaning in Persian, but whether it is also the case in German I highly doubt it.