S.F.
Both of my children have Type 1 Diabetes (formerly referred to as juvenile diabetes). Both blood AND urine are tested and your child does not need to fast prior to testing.
I fervently hope that your kiddo doesn't have T1d, but I am here if you need someone to talk to.
Does your child have symptoms? If so, I would go today and not wait until Friday. If you are just worried because other family members have it (and you haven't seen any symptoms), please don't feel the need to test unless a doctor recommends it.