Congrats in advance!
I'd still suggest a Kr.
Very often you don't need to shoot in 'funny' lighting so you don't have to worry too much about AF in tungsten lighting issue (if its clearly an issue, cause its not a clear cut issue AFAIK). I've never heard anyone in my local Pentax user group that use Kr complain about this issue (its not a small group; every monthly outing sees 15-20+ users gather)
Often enough, you will use window light. You can also use LV AF or MF if necessary.
Save the money for better lenses or lighting equipment (eg. flash
Better yet, save the money for diapers and milk powder
If you are to go ahead with the K5; then the 18-50/2.8 is the one with larger aperture and numerous focal lengths to play with.
A Tamron 28-75/2.8 will do well too, since most baby portraits won't use 18mm much.
Either of these 2 lenses allow some close focus, which is an advantage to get some detail shots.
Most importantly, do research on taking baby photos. There is only one chance and a limited window where they are easily posed (a bit older and they stir easily).
Also research on lighting.
Here are some I took with various lenses and lighting setups
FA35/2 (close to the DA35/2.4) ; Window light with a large white pillow as reflector on the right.
FA43ltd; Flash with shoot thru umbrella
FA43ltd; Flash bounced from the ceiling