Taking possession before gift / usufruct / renting - disadvantages?

...
The question:
Is it permissible (I think so), but is it somehow disadvantageous if the child already moves into the apartment now, that is, before the gift/rental contract, and lives there rent-free for three months?

Does that have disadvantages, be it concerning the tax office, the gift, the valuation of the property for the gift contract, or anything else? Or is that completely irrelevant?
...

...
We just want to be sure and reassured at first whether the child can live rent-free in the apartment for 1-2 months before all the formalities (May/June) and can officially register in the municipality, meaning really move in already. Or whether that might have any consequences/effects/disadvantages later that we do not foresee.
...

In your example at the beginning: Tenant --> You Landlord --> Your parents And later after the gift: Owner --> You Usufructuary --> Your parents Landlord --> Your parents (if not owner-occupied). ...

In the beginning:
Tenant --> You, no, there is no tenant [I]at all[/I]
Landlord --> Your parents, no there is no tenant and therefore no landlord

The apartment belongs to the parents, is vacant, unrented and unoccupied, and no one is registered at the address.

I’m honest, I don’t get it. Why should the child rent their own apartment?

Because someone else, here the parents, have the usufruct on the apartment. Whether this makes sense from a tax perspective (keyword parents' income tax) is another matter. With the usufruct, you can also protect yourself against the child simply selling the apartment (in which you might want to live yourself later). P.S. Since the child is of legal age (it is supposed to rent), one can also consider a reclaiming right on the part of the parents.