Russia was moving Ukrainian children into Russia, that's a component of genocide. German Jews were forced into ghettos, then into concentration camps. The ghettos didn't "prevent" genocide, they facilitated it.
The United Nations first defined genocide in 1948 in the Convention on the Prevention and Punishment of the Crime of Genocide. The treaty outlines five acts that can constitute genocide if they are done “with the intent to destroy an ethnic, national, racial or religious group”:
Killing members of the group
Causing serious bodily or mental harm
Deliberately inflicting conditions of life calculated to bring about the group’s physical destruction in whole or in part
They both serve the purpose of erasing the identity of a people. The methods and means are common between the two: you destroy their homes, you force them to move, you starve them, you kill the ones who don't comply, and you leave the weak and struggling to die. The distinction you're making matters in a UN court, where Genocide has a legal definition and legal consequences, whereas Ethnic Cleansing does not. But that doesn't make ethnic cleansing some preferable alternative to genocide.
If you told a civilian in Gaza "you're not being genocided, you're being ethnically cleansed!" Do you think that would change their understanding of the situation much?