For family and children I like to use the EFS 18-135, it's a "kit-quality" lens but its relatively inexpensive and 18-135 exactly covers any reasonable composition at parent-kid distance.

It's not a fast lens so I also use the $70 YN-465 flash, it's a TTL model so just point it at the ceiling and you have the right amount of nice soft light in any indoors situation.

For my family photos the 18-135 is good enough, it doesn't have the image quality of the 50mm or other high end lenses but it's still good enough (much better than any point and shoot I've ever seen) - for professional and studio work I would use something else but for the kids it's right in my price/weight/quality "sweet spot".

I don't recommend the 50 f/1.4 because 50mm (on a crop sensor) is a bit to long indoors (for me personally most of the photos I shoot of the kids are around 30mm) and it's difficult to photograph children at a wide aperture, the depth of field is very thin and children just don't stop moving so you'll get photos where the child is out-of-focus (so you can't really use the f/1.4 to shoot in low light).