What will happen if a child swallows soap?

Usually harmless. Most soaps are nontoxi. If you feel your child has ingested a significant amount, give them milk or water to dilute the soap. Call 911 for vomiting and diarrhea.
Seek urgent care. Depending on how much soap was ingested it would be wise to seek urgent care.

Related Questions

What will happen if a child swallows liquid hand soap?

Diarrhea. The most common symptoms would be gastrointestinal upset, including diarrhea. You may also get some vomiting. To be safe, call poison control at 1-800-222-1222. Read more...