Once the priority date becomes current, can my child adjust status in the US?

Generally when a foreign national child of a Permanent Resident has a Priority Date become "current" according to the most recent Visa Bulletin issued by the U.S. Department of State, if the child remains in valid nonimmigrant status he may go forward with an application for adjustment of status. There really is no substitute for consulting with an immigration attorney who, after learning all of the relevant information about your family, could advise about immigration eligibilities, options and strategies and offer legal representation in the often complex application process.

I understand that your child's visa expired. The child will be able to reenter with no issues, as unlawful presence does not start to run until 18th birthday.