What are the chances my child will have spina bifida if a prior child had the condition?

Higher than average. I assume from your question that you are relating to a fetus in utero or as yet unconceived. Ultrasound can detect spina bifida prior to birth. For as yet unconceived children, there are genetic causes, such as those of welsh heritage, as well as maternal Folic Acid deficiency in pregnancy. If other family members have given birth to such children, odds of a genetic cause are quite high.
2-5% Spina bifida incidence is about 1-2/1000 so having 1 child with it puts the next child at about 50-fold increased risk.