"Have a good night mom and dad"
Translation:Usiku mwema baba na mama
I'm not an authority on natural Swahili usage by any means, but that might be the natural order that they're mentioned in Swahili. In English we say "mum and dad", "brother and sister", "thunder and lightning", "fish and chips", "black and white" and saying them the other way around just sounds a little odd. It may be that mama na baba sounds odd in Swahili.
Then again, googling doesn't seem to validate this hypothesis, so it may just be an issue with the course. It's still in Beta and needs a lot of work.