If a person has Down syndrome and gets pregnant, what is the chance of a non-down child?

It depends. Women with down syndrome can get pregnant, and often their pregnancies are complicated. If the down syndrome in the mother is caused by one extra chromosome 21, then the risk that she will conceive a child with trisomy 21 is about 50%. However, some down syndrome women may have a translocation - with the extra chromosome 21 attached to another chromosome. Then the risks might be higher, even 100%.
About 50/50. Many females with down syndrome can get pregnant, maybe one half or more. It's estimated that 35 to 50 percent of their children will have down syndrome, as well. Most males with down syndrome are unable to father children.