An opinion:
Handwave all the development issues.
Let's assume that 13 February 1945, the RAF drop a number of incendiary bombs on Dresden. Flying through the night, a few hours before dawn, a trio of B-29s fly across Germany towards Dresden.
At dawn, using the fires started by the RAF as a marker, the Enola Gay, (because if I'm making shit up, let's go the whole hog), drops the first atomic weapon used in anger.
A 15kt blast in a brick and concrete city will* give a roughly 200m fire ball.
Buildings in a 400m radius will be totally demolished.
There will be a pulse of radiation in about a 1200m radius of the blast will kill around 15% of people of radiation poisoning.
The thermal blast will stretch around 2km giving up to 3rd degree burns.
Windows and roof tiles will be broken and damaged up to 5km away.
It won't appear nearly as devastating as the OTL strike on Hiroshima. Basically, it will just seem like a bigger bomb. Not war ending, and would kill fewer people than the OTL Dresden raid. It certainly wouldn't cause the Nazis to surrender.
And, again, my opinion, first generation atomic weapons would just be seen as giant bombs. No deterrence effect.
*Estimations taken from
https://nuclearsecrecy.com/nukemap/