If I was in your shoes, I would go the DA 40mm route.
For the birth of my son, I wanted a fast but not too long lense. I choose the FA 31mm. The room was dim and after the birth I got my moneys worth just right there. I could of gone the Sigma 30mm f1.4 but it didn't appeal to me at all.
Since then.. I have used a mixture of the 31mm/40mm/ and 16-50mm for my baby shots... they all work well.
For your budget and no definitive requirements for very low light work.. I would say the DA 40mm is great... You wife will appreciate how easy to handle your camera is as it is so tiny and she just might suggest you get the 15/21/70 combo to complete your set
Another thought is for the DA35mm Macro... bigger about the size of the 31mm but with close focus. I use DA 40mm with extension tubes when required.
Cheers
Shang