We can certainly continue after this if you'd like but you're correct in assuming that the kidney problem detected on the fetal scan would be a very low indication of Down's by itself. And the fact that the hydronephrosis is mild makes it even less likely that your baby has Down's.
I'm sure the follow up scan will still show nothing significant to worry about.
Let me know if you need to discuss this further please
Your chances I would put at the on in 10,000 level
And actually the scan findings are fairly common and those fetuses turn out to be perfectly normal