Nikon D3200 as backup to D800

I cant speak to the 3200, but the 3100 has achingly slow AF, as well as a half dozen other disadvantages one would expect in an entry level camera. The D7000 (which I use as my backup), while more expensive and fewer MP, is much closer in things like AF to the D800.

The 3100 is small and light though, if you are looking for something for your kids. Put the 35mm 1.8G on it, its lighter then a 24-70.