I spoke to a Canadian in my office. The offer is too low, for a family with 5 kids. Ask for a Grade 16, which would take you to about 25k in basic. Even though all other allowances remain the same, you'd need the extra money for the 2nd car, and bigger villa for the kids.
You'd also need to provision the schooling for your 5th kid, once they start going to scholl at age 3, which is only a year away.