From what I understand, you want a camera that is good enough to catch moving kids
K-r has outlived its life span and there will be a new camera soon (may be with a RICOH label on it
), its only matter of time
Since you are looking at entry level DSLR, K-r scores better than D3100 and Canon T2i (even T3i scores less in Dxo Charts). Canon is good if you are doing a lot of videos but its totally up to you what are your sole need to buy DSLR (to me 1080p 24fps doesnt matter, I want to concerned about pictures).
when it comes to low light, no entry level camera can beat K-r
So you will need fast lens to snap fast moving kids. Although pentax has compatibility with older less, lets be realistic those lens cant be used to catch kids in action, those lens are cheap, good IQ, fast but no AF. The cheapest fast lens is DA 35mm F2.4 and it will cost you anywhere between 175-200$
On the other hand Nikon and Canon does have cheap, fast, AF 50mm f1.8 lenses starting from 135$
So here is the catch, All Nikon and Canon 50mm lenses are not image stabilized, whereas Pentax has in body stabilization, so all the lenses has Image stabilization (yay!
)
K-r prices will drop once a new model comes out in the market. You can wait till then and miss all those precious moments or buy it now
P.S : touch wood, but i havent seen any FF issue with my camera, I now own Tamron 18-250 with Kit lenses (if you want I can sell you those lenses to you
)