What is the probability of a child getting lupus if a parent has it?

Unlikely: While there is a degree of hereditary predisposition to lupus, the risk of a child developing it in this situation remains low.

Fairly low: Systemic lupus erythematosus is a complex genetic disease meaning that there are many genes that may affect risk(over 80 known so far) and only a handful of these genes is likely to have a change that increase risk for lupus in any one person. An environmental component is also a major factor (often virus or other unknown triggers),In identical twins, if one has lupus most often the other doesn't.