I have a mare that recently reached level 50, so now her foals are gen 1 Descendant of Heroes BUT are her previous babies Descendant of Heroes too or not? Also is there something somewhere that says that pet is a Descendant of Heroes by whatever degree? If not I think something should probably be added even if it's only a red number somewhere.

No, her previous babies are still normal unicorns. 

And as I mentioned in your suggestions thread, Descendant of Heroes is the same as Genetic Power. Your lvl 50 mare now births Genetic Power 1 foals. And this you can easily see on the profile page of your pet. To show you what I mean, I took the liberty of marking on your Warrior Princess that she is a GP1 (or Descendant of Heros in 1 Generations).

OK, now I get it, have another question though, if the mom is level 50 & the dad has Genetic Power: Grade 3 shouldn't the baby have more than a Genetic Power: Grade of 1?

No, you can see that part in the FAQs:

... "If parents has a difference of 2 or more grades of Genetic Power, the child won't get Genetic Power grade for parents' genes, it can get only 1 Grade for parents' levels."

So that means the baby in your example will get GP1 from the lvl 50 but not from the GP3 parent.