That is why I sold me xxbis and bought a xbis instead. It is a substantial difference from f4 to f2.8. I took most of kid's performance pix with xbis, f2.8, iso 800-1600. You can not use flash in most of the situations. It will be a good investment because you will use it a lot.
For new parents, the babies won't perform serious indoor shows / activities at least for a couple of years. By then, the APS-C bodies' high ISO outputs should match the current full-frame ones, so even at f4, 1/1600S shutter speed will be quite usable?