It is understood that upon this realisation, the two girls were not placed at the home.
Southern CYF regional director Kelly Anderson has since apologised to all of the affected parties for the mistake.
"It is totally inappropriate that arrangements were made to place these two children in that placement for the weekend,'' she said.
"The children's mum and grandmother are rightfully angry and we have apologised to both.''
She said appropriate action had been taken to ensure the incident was not repeated.
"While thankfully no harm was done, it is extremely concerning that this occurred. It falls short of what we expect from our staff. I have formally spoken with the staff involved.
"I have looked into how this happened and as a result introduced new rules and checks to ensure this cannot happen again.
"We have put in place a robust respite plan that they are now confident is safe and appropriate. We will support this grandmother fully to care for her grandchildren.''
The new plan is understood to currently be in action.
