回复：我明白你说的 ：）。 如果是我，我会写封信问，要不然心里不踏实。

They had better not take advantage of this kind of the CBP officer's mistake to avoid any potential trouble in the future. Your parents had better visit the CBP local Deferred Inspections Office to fix their I-94.

See this website to find the CBP Deferred office in your area:

http://www.cbp.gov/linkhandler/cgov/toolbox/contacts/deferred_inspection/deferred_inspection_sites.ctt/deferred_inspection_sites.pdf