Age 11 is plenty old enough for a DC to be allowed into town with a friend, unless they are spctacularly scatty.
As you have said your DD is very sensible so really no reason for your DH to object.
A precedent has been set with your DS1, so yoyr DH knows he is BU. That is why he refused to discuss it.
My DH is like this, says "what if she gets snatched off the streets" - this is in a friendly little market town, not the Bronx by the way!
My answer is, well I guess she could be kidnapped off the streets by aliens, but if we follow that logic she will never leave the house.
I think it is because she is a girl as I don't think he will have those worries about DS, though he is a bit young at the moment and is as scatty as they come.
I am not letting him get away with it, though, as DDs need to learn some independence.
They will have to leave home and fend for themselves on the streets eventually and it is better that they learn to look after themselves gradually like this rather than be turfed out when they go to college at 18 with no street smarts whatsoever.
Now THAT would REALLY worry me.