Well, at f/4 with 50mm focal length and a focus distance of 4 metres the depth of field is about 1 metre. If you focus on one of the kids who is at 4 metres the other one only has to be a bit more than half a metre closer or farther away and you are out of the really sharp zone. For the same settings but with the focus distance of just 2 metres the depth of field is only around 60cm so focussing on one kid means the other has, ideally, to be less than 30cm closer or farther. If you must shoot at f/4 then you probably need to focus manually halfway between them or, if you are nimble enough, use AF but then recompose and move yourself forwards or back as appropriate half the distance between the kids.
Try shooting in Av mode at f/8 and, if the shutter speed gets too slow and you end up with motion blur, bumping up the ISO to compensate. You might end up have to do a little extra post-processing to reduce sensor noise but that is a better option than starting off with an out of focus subject.
Maybe maximising depth of field in family portraits where flash isn't an option might be a suitable topic for another of Gordon's DSLR Tips workshops?