To put it bluntly, it isn't that dangerous if it is done right. What I mean is if it happened at a more mature age, when the cat was ready, after both cats were tested for genetic/hereditary problems, in a controlled environment, with two purebred cats who were part of a breeding program.
The problem with a cat being out there able to mate with any intact free roaming tomcat is the fact that intact free roaming tomcats are the number one carrier of all sorts of diseases. You have Feline AIDS, Feline Leukemia, respiratory infections, fleas, ear mites, worms, etc. She could now have any of those things and/or could be able to pass it down to her babies.
Plus at such a young age, she may reject them, she may eat them, she may kill them... rejecting them means you will have to bottle feed them. This may sound fun and cute but not when you have to do it every 2 hours all day all night...
There is just a ton to think about. It really is dangerous. On the other hand, everything could go perfectly fine. It is a crapshoot and not one I would be willing to take. Plus then you would have to vaccinate and deworm the mother and all those kittens, and then spay and neuter them all and then try to find them a home... A LOT of work!