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.
The construction behind
Millionen syrische Kinder
is called "Apposition mit Kasusangleichung".
Millionen syrischer Kinder
would be "partitiver Genitiv (Genitivus Partitivus)". Both constructions are grammatically correct and denotate very much the same thing (1 million of Syrian children), but they have a different grammar. For more information see de:wikipedia: Apposition and canoonet
I came across the same thing a while ago when I saw ein Glas 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.
The question should be why Genitivus Partitivus was permittable at all. Kinder are in principle countable, and in subject position it will always be zwei syrische Kinder; It's the indefinite Millionen that's tricky. The casing depends on whether you want to stress uncountability or not; also, if definitely uncountable, e.g. Millionen grüne Liter Asbest (nonsense), would in my humble opinion more likely continue in active voice.
The nonsense-phrase "hier werden sie geholfen" (hier wird Ihnen geholfen werden) comes to mind, because it's the logical continuation (though whether that's etymologically true is a different question) of the mish-mash that you get from
Millionen Syrischer Kinder wird geholfen
as it's uncountable to boot, it has to take the singular flexion. Substituting a noun phrase that commands plural inflection, like the pronoun "sie", we get
Hier werden Sie geholfen