Hahaha, I am a new father to my lovely twin sons and your comment is as sincere as it could be. I super agree with you.
When it comes to child photography, big aperture and fast shutter speed is a must. With respect to focal length, I really dont think 35~50mm is good enough for taking picture of children. From my experience, I find 50mm on my APSC body is just right enough. 50mm give me a head size of 40~70 % of the photo and that is important since everyone wants to see the detail of child's skin tone and texture of the skine.
In fact, I sometimes use EF 85mm F1.8 USM to take portrait shots of my twin sons. this particular lens gives me more detail of my sons' facial expression. Sigma 50mmF1.4 is used when I want both of my sons in one picture.
Taking pictues of young infants under age of 3 is as challanging as courting young woman. right equipments does help.
Another great lens for taking child photography is Tamron A09.
Anyway, before this post gets too long, I will share my opinion of best lens for taking child photography:
Indoors: in no preferential order 1. Tamron A09 28-75mm F2.8 2. Sigma 50mm F1.4 HSM 3. EF 85mm F1.8 USM 4. Tamron B005 VC
Outdoors: in no preferential order 1. Tamron A05 70-300mm VC USD 2. Tamron A09 28-75mm F2.8 3. EF-S 15-85mm IS USM 4. EF 85mm F1.8 USM
If you check the list above then you will find most of the lens are not expensive at all. In fact some of the lenses are cheap to the bone in terms of qualities it delivers from the lens.
Taking pictures of new born child and lovely wife is the happiest thing I can think of after getting married.