B2 Tourist Visa - Extension of stay

Hi,
I applied for my parents- B2 Tourist visa extension of stay just now before 3 weeks of their i94 expires. Documents delivered to USCIS office per Fedex tracker, but waiting for receipt notice. If receipt notice did not come before i94 expiry date, can they still stay in US as application is pending or need to leave before i94 expires?

Thanks
Praveen