If a father raped his own kids at the age of 4-5 are the kids doomed to fail in life?Are they damaged forever?

No one is "damaged forever".

If a patient works very hard with a good and supportive therapist, they can recover and go on to a highly successful life.  Believe me, people can and do recover.

What does it say for the "father"?  It says that he is a criminal who belongs in jail.

Generally kids don’t really recover from that sort of thing. But some can certainly beat the odds. 

Obviously the father is a very sick man and a criminal.

Some children are more resilient than others. They deal with trauma more successfully than others. Exactly how someone suffering this experience will turn out cannot be predicted any ANY certainty. My sister, who is a mental-health counselor, wrote her master's thesis on this subject, so I am familiar with the issues.

I know of some people who suffered rape as children, at the hands of relatives, and who turned out to be quite successful in life.

--  A girl is not physically capable of having vaginal sex at age 4-5.  A European girl needs to be at least 7 or older and a (smaller) Asian girl needs to be over 8 for her to have vaginal sex, taking the length of a adult male penis, without suffering any physical damage to her body. Under those ages, a girl may suffer from a "fistula" (a hole between the vagina and urethra)  causing leaking of foul smelling urine from the vagina. She could even die.
--  Anal sex with either male of female by an adult could happen with or without physical damage.
--  Oral sex by either child on a male would not cause any physical damage to the child.
--  All forms of sex on a young child can, and probably would, cause mental and psychological trauma on the child. Whether the child survives, and has no problems in the future, would vary, but certainly it would have some affect upon them and their relations and interactions with people in later lives.

no i dont think they are doomed i think they may need some counseling but will probably look down on men.....

C-PTSD

BPD

Are probably the two most common.  The kid would have issues with repressed and many other things.  It literally changes your neurobiology (brain size, shape/structure).  It just makes changes you can not really undo. Though you can manage."dead man walking"

They are victims & should never feel guilty or ashamed. With proper therapy they should live normal lives. Should not feel a chip on their shoulder.

Sometimes yes, sometimes no. It all depends on the child. Mayn will turn into drug addicts and alcoholics because they are not getting the proper therapy. Then you have the one child that is the strongest of them all and will persevere over the rest.
The "father" should be jailed for life. The "father" is a sociopath.
https://www.wikihow.com/Spot-a-Sociopath

I was raped from 6 until 20 I am now 23, I never got over it, still suffering and I hate men (9 men raped me, throughout the years and in 2018 got raped by all 9 they drugged me and all did it), never moved on.