I think u may have an interview email soon. Ur interview date may monday. If u can show ur bank account from ur mobile phone at interview and u have good money evidence u may get visa on monday
sponsorship letter is about u have to pay all the cost of ur childrens. And family certificate is the list of ur family members approved by goverment(some other country may different).
Process is u have to apply to get visa first. If u have visa. U can apply dtv dependent visa for ur children. U need birth certificate of them and family certificate and a sponsorship letter for them and bank statement
and there u have to choose the embassy u want to apply. Example if u are now at HCMC. U choose the HCMC. If ur Hanoi u choose Hanoi. If Laos u choose Laos
U have to apply to get DTV first. After u got dtv. Ur spouse and children can apply under ur dtv visa number. U need marriage certificate for spouse and birth certificate and family certificate for child. U also need bath