yeah the 5DII and kids don't mix well (the focus can't track well enough.
If you want a good kids body, go for the D700 IMHO. Much better subject tracking.
BUT that would mean a switch.
I rented a D700 the third time my gear was back to Canon. The subject tracking AFAICT only works when all 51 AF points are active, and for me it was lagging at least half a second behind motion. I guess since the 5D2 doesn't do any tracking at all this is technically better, but still not useful.
I have some priceless shots of my son thanks to my 5D2. Now that he's mobile I'm shooting often with my 85 f/1.8 for the AF speed, and indeed sold my 50 f/1.4. I miss the MFD though.



It's more for very unpredictable movements (

