Interesting facts from Genetics 101: for every gene, there are two alleles, one allele from each parent. An allele can generally be dominant or recessive. Two recessive alleles make a child takes on the recessive appearance. Two dominant allele, or a dominant and a recessive gene, make a child take on the dominant appearance because the dominant allele covers up the recessive allele; when that child grows up and has their own children, they can still pass on the recessive allele An allele can also be incompletely dominant, which means the dominant and recessive alleles blended so that the child possesses a mixture between the dominant and recessive appearances.
The gene for brown eyes is dominant over the gene for green eyes, which in turn is over the gene for blue eyes. A green-eyed man and a blue-eyed woman have a high chance of having blue-eyed children, which is what happened in Frozen. However, a brown-eyed man (as Jack is naturally) and a green-eyed woman can both carry the blue-eyed allele. Jackunzel would be less likely to have two blue-eyed children than the girls' canon parents, but then again, it's possible that Elsa's ice magic also influences her looks, so one naturally blue-eyed child isn't too much of a stretch.
The gene for brown hair is completely dominant over the gene for red hair, which is incompletely dominant over the gene for blonde hair. A brunette woman like Anna's canon mother can carry the ginger gene; Anna got one blonde allele and one red-head allele, and they mixed to give her strawberry-blonde hair. Jack and Rapunzel both have brunette hair naturally, though their respective magic powers make them look white-haired and blonde. One of them could carry a blonde allele, and the other a ginger allele, and pass those on to Anna. And, of course, Elsa could get her magic from Jack, so it's possible that she's a "dyed" white-blonde as well.
Long story short, if you try to argue that Jack and Rapunzel can't have children who look just like Anna (and Elsa)...well, technically, it's entirely possible that they can. Take it from a strawberry-blonde, blue-eyed girl with a brunet, brown-eyed father and a brunette, green-eyed mother: I know of which I speak.
Now, getting around the fact that young Rapunzel and Eugene were invited to Elsa's coronation? Yeah, not touching that. My point is that it's possible for Jackunzel to have children that look just like Anna and Elsa. Anyone trying to say that Frozen Tangled Guardians "can't happen" because of Anna's reddish hair needs to get a new argument.