Ok, before all this, frankly, I don't think you are much of a Pentaxian with those lenses
Your major ones like the Tamron zooms will just form the same value/performance argument on whichever brand since they also exist on those mounts.
I've had m4/3, Canon FF.
Still using Sony FF, Sigma Foveon, Pentax
IMO, All systems can be made to be rather cheap and good.
Canon 5D
EF 20/2.8 (or even an old cheap Nikon AIs 20mm with adapter); 28/2.8; 50/1.8; 85/1.8
These for a cheap system, that works.
FF too.
Get a used 6D if one has the money and thats a nice system.
m4/3
EM5
12mm, 25mm, 45mm, 75mm
Not that cheap but the lenses/system is small, light and fast aperture.
2x crop factor does give more DOF for the same FOV though.
Sigma DP1m + pano head; NEX5n with 50/1.8 (or accompanying 35mm/50mm camera of choice; I use a Q7)
Light setup
Not expensive
OMG! level resolution when needed (from the DP1m)
Flexible shooter from the other camera.
There are just so many cameras out there nowadays.
All are capable of taking good photos.
None are perfect (though Pentax certainly stands out for the wealth of features and well thought out layout and usage flow for their cameras)
The thing about 'Needing a camera with good AF to take photos of my kid" , IMO, its more a photographer issue that the camera.
Moving kid in poor light will tax any system.
And think it over.... how often is a kid running from point A to B really that worth a shot anyway?
Often, imo, its when they are more stationary that they are interacting with the environment, other people/children, doing something.
Now those are more worth shooting.... (easier too with any camera
)
Of course we'd still want some shots with the children moving about, just not as many, MHO.