I think it would have been nice had you been given a quick message heads up that the mother who you were familiar with was ill, so the father would be coming by with the young child.
Many play dates with young children is just a drop off and pick up, but if in this case the parent was staying to supervise/help, well that's more hands-on than this to drop off isn't it? It's great to see a more involved father.
Would you have preferred, if you'd known that it was a man who you hadn't met yet, who was going to be dropping off the child and staying, so that you could have made the play date in a public place? Well that's up to you. I don't think it would have bothered me.
Whenever I end up with someone coming to my house for whatever reason, whether it be a service call, or someone dropping by, I have one or two people who I can message who I trust, and I send them a quick note just to say "Oh, 904 a.m. electrician has arrived, they said his name was going to be Tom, he's from XYZ electrician company."... Just so that somebody else knows that somebody else is here what time it was, etc because I live alone.
Aside from that, I wouldn't have worried about the dad and the play date but that's just me. But yeah, a quick message wouldn't have been a bad idea just to let you know. Of course, if they had sent you a quick message just before they were arriving, what would you have done? Cancel the date? Quickly tried to move it to another venue? Set yourself up into a worrying panic? So I'm not sure that a last minute update to you would have been very helpful in your case if you're not comfortable with men you don't know being nearby in a private place