My child is visiting my brother in US under VWP and he didn't know that allowed time is 90 days, so her return ticket is with 18 days overstay

Hi. He will not be able to use the Visa Waiver program anymore.

It is a bad idea to overstay under the visa waiver program if the individual wants to reenter the US at a later point. The person can then have problems with CBP upon entry. I strongly suggest that you forget the cost of the ticket and buy another so that she does not overstay. Due to the limitations of the Lawyers.com Forums, Alan Lee, Esq.'s (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.

You will have to decide whether avoiding that cost of buying a new ticket is worth the consequences.  Overstaying your visa may result in being placed in removal proceedings, though that is unlikely unless you are detained by law enforcement for some other reason (such as a routine traffic stop).  Overstays less than 180 days have no direct consequences, but when evaluating your risk for overstaying in the future (such as when deciding to issue another visa), the overstay can be taken into account and your visa application may be denied.  When you try to enter the US later, even under the VWP, the border patrol may refuse you entry because of their concern that you would overstay yet again since you did it before.  You may be able to make a short trip to Mexico or Canada and re-enter to start a new 90-day period of stay and then return on your originally scheduled flight.