Where did you upload your documents in the first place? Then do something similar in your children's profile. I suppose you can also submit a brand new visitor visa application for the whole family (you, your husband and the children).
Hey there we are also experiencing the same issue please help me and my husband got visitor visa approved while I was pregnant only we applied we thought it would come before my delivery but it didn't then as soon as our baby born we try to add him but we couldn't because he born as preterm so he was in the hospital for 1 month then after that only we apply for his passport but while waiting for our baby's passport our visa approved so we applied the baby separately but in our same Ircc account we applied new submission we didn't create an account we still didn't receive my baby's visa it's being 5 months now