I wouldn't even bother with them, no matter how good the parents temperaments are. You don't know ANYTHING about the rest of their families and neither do the owners. Maybe the mom is the only calm dog good with kids out of the entire family tree. After all, Presaras are intended to have a certain level of human aggression. You don't know about the health of either parent either. Rotties aren't supposed to be anywhere NEAR 150lbs, so either that owner is exaggerating the weight trying to sell "bigger is better" or he is an extremely poorly bred dog likely to have a limitless number of health problems. Either way, not someone I would trust to add a member to my family. Rotties are also rife with temperament issues, including aggression.
If you are going to get a mixed breed, get one from the pound or rescue. Don't reward clueless owners with money for their trouble. All that does is encourage them to do it again. :(
I'm not sure why everyone thinks that mom and dad are the only ones who have an impact on the pups. Sure, they have the greatest, but their genes come from THEIR parents. Just like you might have a son that looks just like Great Grandpa's brother Frank, dogs can have issues pop up from several generations back in the family tree. That's why good breeders are so important. They know the entire family, even brothers and sisters both for health and temperament.
As for the RR, again, GOOD BREEDER. Have I said that before?? Not all pups in any litter are going to be identical. A good breeder will tell you if one of their dogs would work in your family or not. By the time they send your pup home at 8 weeks, they will know that this dog has a lower prey drive. That is why a good breeder won't just let you hand over $$ and pick out whichever cute pup catches your eye. They've been with these dogs every day of their lives and know their personalities better than anyone.
No matter what breed you get, training training training to be around livestock. ESPECIALLY chickens. Of course, if you plan to have the dog off-leash hiking with the kids, that isn't the only prey drive you'll have to worry about. You're going to need the help of a good trainer to "critter proof" the dog as well as a perfect recall. Dog comes back 100% of the time, no matter what exciting thing he is doing.
I'll say again, protection from a large dog is 99% visual. Anyone who is going to approach with ill-will is going to already have a plan to take the dog out of the equation. They either have a gun or a knife and just plan to kill the dog and go about their crime.