I shot for years with a Pentax (manual focus) with a very heavy 35-105 f3.5 lens hanging off if it. While it took some fantastic baby and child pics, I always had two points of dissatisfaction with it. 1. Too heavy, and 2. not wide enough. You tend to shoot your children at very close range, and while the baby may not be crawling yet, the child will soon be quite mobile. At that point, wider will definitely be better, and give you greater flexibility. So, IMO, 24mm on a reduced-frame camera is not enough. 19mm would be the minimum on the wide end.

The weight issue can also become a factor, walking around zoos or whatnot with the child, or even using in the backyard. L lenses are great quality (like my old Pentax), but they take it out of you. ;-) I've only ever once slung an L lens from my ancient D30, but I was amazed at its weight and bulk. I was comparing it, though, to my Tamron 19-35mm.

In fact, that's the lens I would recommend, myself. It does have downsides--3.5-4.5 is its biggest weakness--only one, now that I think about it. Optically, at least on the cropped-frame, it is excellent. And build-wise, it is not so bad as you would think, considering its price. Unlike the cheaper Sigmas and Canon lenses, no plastic mount. It is light, but feels solid. And sheesh, you can buy probably half a dozen for the price of one L lens. ;-) And one advantage of it over an EF-S lens is that if you ever want to strap it onto a full-frame camera. I will work--can't vouch for the image quality at full-frame, but it certainly won't be the worst for distortion. ;-) (and, for the record, I've been surprised at the lack of lens flare on this lens).

Also, at its price, you can get two--one for each camera.

One other downside is that it's not a long lens. It would be advantageous to have more length. Which you may want on one lens. Personally, I find that the vast bulk of my photography is with this lens, though, and I use my longer lens only on occasion.

Those are my thoughts--the thoughs of a nobody photographer, but in a similar situation, goal-wise. :-)

-Jon