Is this correct? Because then people will not register anyone with a child or of child-bearing age. Maybe it's correct; I don't know. Just a little surprised.
The second thing is, even if children are difficult to unregister, wouldn't they still be able to unregister him?
I don't know the answers to any of this, but these questions came to mind.
The thing is, it's pretty difficult to get permanent registration somewhere for money being a Ukrainian citizen, for these and other legal reasons, so landlords usually offer temporary registration (which is fine, because for a Ukrainian citizen it doesn't really matter what type of registration you have, you can enjoy the same rights with temporary registration, and you can even live without any registration whatsoever).
However, with foreigners, you must get permanent registration for PRP, temporary won't cut it, but it's pretty simple to unregister someone, especially a foreigner. However, I'm sure most landlords don't expect a foreigner to have Ukrainian citizen kids, and certainly don't expect them to register the kids with the foreign parent (generally, parents tend to register the kids with the Ukrainian parent). So basically, they don't expect this situation to occur (due to very low probability), and there is no way really for them to prevent this from happening, because they can neither check if you have any children already, nor ensure that you won't have any children after getting your registration at their place

It is possible to unregister a child, but it's a huge hassle (convincing the court and also getting permission from CPS), not to mention that there's a real chance of being denied by the court and/or CPS if the landlord can't prove that the child has some other residence in Ukraine where he/she could be registered instead of their place.