There should be no issue with her marrying here in the US while in B1/B2 status but that does not mean she or her dependent child can stay in the United States as a permanent resident. I would encourage you to discuss your plans with an experienced immigration attorney to avoid common pitfalls.
If you have any questions regarding immigration, please feel free to call me, immigration attorney Shawn Mesa at (813) 679-5780.
Disclaimer: This information is provided as a public service and not intended to establish an attorney-client relationship. Any reliance on this information is taken at your own risk.