score:4
B1/B2 visa applicants need to overcome the US assumption of immigrant intent. If your mother-in-law portrayed ties to home that included her 12 year old son remaining behind during her visit in her application, it would be unwise to present a different picture subsequent to obtaining her visa by applying for a visa for her son to accompany her. It could well look like an attempt to hide intentions to overstay in the US; having adult daughters who work stay behind isnβt such a strong tie as a dependent child.
You will never know if she would have been given the visa if she had applied with her son in the first place. IMHO it would be safer to follow the advice of the Immigration office in Jordan and proceed with the visit as described in the application. You should be aware that even having a visa does not guarantee entry https://travel.state.gov/content/travel/en/us-visas/tourism-visit/visitor.html