I think that you have a difficult choice, and your final choice might be linked with your budget.
I would recommend possibly 3 options:
* K-x,
* K-7, and/or
* new good lens(ses).
The K-x is a nice entry level dSLR that is very good value. The K-7 is a semi-pro level dSLR with nice WR, if you shoot outdoor, and it is good value right now, with several 2nd hand bodies on the market. Either way, you cannot get wrong with either camera bodies: these are nice camera bodies.
However it is also worth to invest in some good lens(es). "Good glass is worth every penny" and that is very true. I have a K-7 and I can see the IQ difference with the prime lenses.
In your case, I could possible suggest to consider a fast prime (around 50mm and f1.4). It would allow you to take shot indoor of your little fellow without flash. I have also young children and I found that my shots are more natural when I do not use the flash. This requires however a large aperture lens.
Hope that the comments will help...