Visa interview appointments

I paid visa application fee for my lil ones on this 12th but i could not book an appointment for visa interview. It has been more than an year i see my 4 & 2 yr old. Please someone guide me, how to book an appointment. Their DS 2019 is going to expire this 31st. I need to book an appointment for them before that. Please guide me if any knows.:pray::pray::pray::pray: