Technically, if a child is not collected and there is no-one else who can (perhaps other relatives), then we are supposed to contact Social Services. In this particular circumstance, you knew that the dad was on his way, and I don't think that SS would be too interested if you'd rung.
I think that, in most circumstances, they would listen to you, and tell you to keep the child rather than put them into temporary care!!
I had children later than 7pm a couple of years back when all the parents got stuck in a county-wide blizzard. They were all on their way, and it wouldn't have occurred to me to ring SS for that. In fact, most of my childminding friends were in the same position - if we had all phoned SS, there would have been mass panic (and no-one could drive anywhere anyway!!)
I have got written permission from parents to hang onto their child if there was an emergency, which my network co-ordinator has said is OK.
Several of my minding parents don't have family nearby. The parents have said that if, God forbid, they were involved in an accident and couldn't contact me, at least their child would be in a safe and happy environment, with someone that they love! This is one of those "worst-case scenarios", and I do have phone numbers for family members in an emergency, but some of them would take hours/days to arrive.