Presas were not bred as livestock dogs. The breed was developed as a man fighter and their job was to track down runaway slaves, and not too gently.
Probably a very well bred Presa from a very serious show hobby breeder who concentrates on breeding for good temperament, would be OK with kids. However, I would be more than extremely careful about where I purchased a Presa. A mixed breed, who by definition is bred by an irresponsible person, would not be where I would look for a dog with Presa genetics.
I feel the same about Rotts. They are very good dogs when well bred, but there are a lot of Rotts who should have never been brought in this world by people who are only interested in getting money for puppies. A well bred Rott from a serious fancier who is devoted to the breed and breeds for temperament would be suitable for children. A Rott from a different type of source would be too much of a risk.
Anyone who has deliberately bred a Rott to the Presa is probably intending to produce a fighting dog or an area guard. Unless you have marijuana fields,. you have no use for an area guard.
For protecting children I like the large herding breeds (not the drovers). A well bred Dobermann (again, be careful where you buy) is excellent for children. The very best dog you can buy to protect a child, that will not make a mistake in judgment, is the Leonberger.
I suggest that you pass on the Presa.