I was sceptical of this claim so I did some research - 700,000 is almost certainly too high, but other than that it's disturbingly true:

The 700,000 number comes from a Russian parliamentarian in 2023, and refers to orphaned and abandoned children Russia has 'protected' from conflict zones in Ukraine. A later Russian report walked this back a bit, and claimed that most of this number were children accompanied by family voluntarily escaping the fighting by feeling into Russia.

Obviously we should be sceptical of what Russia says about this, but this is not the same number as the number of children abducted - not even Ukraine alleges it to be this high.

The number of children abducted and forcibly deported was officially reported by Kyev to be 19,000 to 20,000 at the time of the above claim based on the data (nearly 30,000 now). The real number is almost certainly higher - many Ukranian officials believe the actual amount is higher, with one saying it may be into the 'hundreds of thousands'. A US report in 2022 estimates that Russia has "interrogated, detained, and forcibly deported... 260,000 children, from their homes to Russia"

Even if we take only the low amount that can be fairly positively stated as abductions, that's nearly 30,000 children. Various reports have shown some of these children being given new Russian identities and false birth certificates, and being put up for adoption in Russia. Some have testified to being indoctrinated and shown pro-Kremlin propaganda.

This broadly constitutes Cultural Genocide - whether it technically is or not is for academics to argue over, because the legal definition of genocide is complicated and so much is unkown.

Whether or not you want to call it a Genocide, it is undeniably a War Crime. The ICC has issued arrest warrents for Putin and Russian Children's Rights Commissioner Maria Lvova-Belova over this.