Consider Plato's idea for how (Guardian) children ought to be raised in the kallipolis and all the objections Socrates' interlocutors find with it. Without a nuclear family, Plato thinks children develop familial attachments to their fellow citizens instead, binding individuals to each other and the state in new ways. The myth of the metals serves not only for class distinction and justification, but to convince citizens they are brothers and sisters due to their common mother, the Attic dirt.
As regards your specific questions:
How ethical do you think it would be?
As for my own thoughts, well, Imma suspend judgement on this non-evident matter, but here's what Plato had to say:
Would it be morally acceptable if those kids were raised with no mental problems and perfectly suitable for adult life?
Not only is raising children in this way morally acceptable, it is, in fact, the ideal, most virtuous way, since we are making justice in the individual (and therefore justice in the state) essentially idiot-proof.
Are there any psychological/philosophical problems if you were not raised by real parents?
You can ask any adoptee about this, if by 'real' you mean biological. If you use 'real' to mean a set of nuclear parents -- a mom and dad -- then Plato would say no -- psychological problems arise when you are raised by flawed parents, not through the perfect guardianship and programming of the philosopher-king state.
Do kids need that maternal instinct of unconditional love or that's
just a myth?
Myth or no, Plato would argue that the state is better positioned to meet this kind of need.
Would this be an step towards a "Brave New World"?
Some people consider Plato to be a proto-fascist, so yes, maybe.
What if in the future because of the low natality there is no other
alternative than this to keep society going?
Plato has thoroughly, if not convincingly, addressed the breeding issue.
Would this be a problem for religious people for which a father and mother figure is important?
Yes, I think the neo-Platonists of the Middle Ages/Renaissance struggled to reconcile Christian thought on parents (and their real/symbolic importance) with Plato's guardian class husbandry and communal child-rearing approaches.
What if societies of the future evolved into this because of the equal
roles of women and men and they found to raise kids difficult, lengthy
Yes, Plato also addresses this, which is why some people consider him to also be a proto-feminist.
Why old people´s homes are accepted but a "baby's home" could not?
Isn't this just like just a social taboo like gay marriage or gay
parenthood was years ago? Perhaps is the bad reputation of orphanages
in the past which makes this idea sound creepy?
This is an excellent question. There is no reason why group homes for old people ought avoid similar disrepute as group homes for children -- abuse, neglect, and a thousand other human evils are certainly visited upon some of those people, too. Maybe we just care about old people less. Babies and children are cute and seem to have promise, whereas old people, well, I think we (as a society) often just wish they would die already or otherwise go away. Plato thinks old people who have aged out of the breeding program ought to just be able to enjoy the small pleasures of a life well-lived, at the state's expense.