You have to abandon the literality of this requirement:
The children can be mix of the parents gene
because there is no way of doing that, unless the creature has a complete genetic reengineering laboratory onboard. Just think that some creatures have a given number of chromosomes and genes, and other have different numbers. What would, or could, our xeno genes' be coding for? (As @Pelinore points out, this is what Orson Scott Card's Wyrms do - they reengineer a new copy of DNA holding both the host's and the guest's code entire, then use the host's, or a rearrangement thereof, to build the body).
Also, to be able to breed with other creatures you need the equipment. Pigs have corkscrews [thanks @Separatrix for the correction], ducks have worse, other creatures have (detachable) hectocotyla, sperm packets, fixed-shape and variable-shape phalluses, and so on. Then there's the matter of size. And then there are egg-layers. There is no way a single being can be physically compatible with more than relatively few other different species.
So we find an alternative. Turns out there already is one.
When you say "the children's genes" you're thinking of a child organism. But the child organism is a complex structure, and by rights its whole genetic code does not come all from the parents. By mass, perhaps four fifths do. But the rest is made of commensal organisms such as gut bacteria or tiny skin parasites/symbiotes.
Also, while 50+50% of the core DNA comes from the parents, the mother also contributes mitochondria. These are "sister" organisms with different DNA that penetrated our cells many million years ago, and instead of being digested, struck an "agreement" - they would supply a more efficient way of producing energy, and in exchange they would be able to dwell inside our very cells and take what they needed. Food and lodging, in other words.
In a healthy organism, mitochondria outnumber human cells by more than one thousand to one.
We are now already a hybrid of "human" (48 human DNA chromosomes) and "mitochondrion". Only we've grown so dependent on our guests that there can't exist a mitochondria-less human - they would die in very short order (cyanide poisoning does this).
So, what has happened once might happen again.
What our "creature" could do is infect another creature's cells and either replace their mitochondria or live together with them. The creature would also live in the host's neurons, as they are cells like any others, which means that the result could be as much "creature" as "host" - their joined mind would co-evolve if infection took place upon conceiving.
Intra- and inter-cellular communication would allow the xeno organism to be a single organism instead of a collection of disjointed cells.
The hybrid would breed true - a twist could be that the creature could trigger parthenogenetic pregnancy, meaning that a female could have sex with another female and get her pregnant, "half" the baby's inheritance coming from the human mother which would give birth, and the rest from the alien.... "mather"?'s xenochondria.
The difference shown by a hybrid might be none to all of the following:
- different physical strength and intelligence (both more and less could be justified).
- different tolerance to poisons and toxins (ditto, but very probably in the same direction as above - i.e., either stronger and more resistant, or weaker and less resistant)
- different tolerance to heat, cold, fatigue, hunger and dehydration
- different pregnancy duration
- phenotypical changes, such as skin color and texture or body fat and proportions
- sensorial changes (e.g. better/worse tactile sense)
Infection of an adult would probably only entail the first three set of changes, and possibly behavioral changes, even radical ones up to and including shuffling around murmuring "braaaiinnnnss....", when the infection reaches the frontal lobes. Or it could also of course mean death.
Some stories relating to this are Brin and Benford's Heart of the Comet (1986) with Saul's cyanutes, and especially Walter M. Miller Jr's Dark Benediction (1951); both describe this kind of endosymbiosis. A more parasitical kind, ending with the host's demise, is described in Greg Bear's Blood Music (the short story).