Hi David, a little insight on the Canon 75-300 from a long time Canon Shooter. First let me say that it isn't a bad lens at all however it has its short comings. Kind of like comparing a Bear Whitetail Hunter to one of todays Mathews bows.
The actual images are going to be fine for most applications especially if the subject isn't moving. You do mention Kids Sports but don't say what sports and in what lighting conditions your wife will be using the lens. First strike on this lens is that it is slow to focus, the newer 70-300 is much faster but that comes at a price.
You will also notice a little bit of softness at the 300mm end of this lens. Depending on what you do with the images that might not be acceptable and may even be frustrating for your wife since she won't be getting those nice sharp images you might be looking for.
Don't get me wrong, if the subjects are still or slow moving and you have plenty of light, this lens will do a pretty good job. Of course the person behind the camera has a lot to do with the results.
Now for alternatives, and price may be a determining factor here but I will tell you that the better lenses are much better investments and have a higher resale value down the line than the kit and entry level lenses like this one. You will find that the better lenses hold their value and as Canon and everyone else increases their prices your higher value lenses might even make you a couple of bucks if you buy them right. There are a bunch of sites that you can buy slightly used lenses for 20 -30 % off new, use the lens for a couple of years and then resell it for what you paid or maybe a little bit more.
If your kids do inside sports the 75-300 just won't work unless you are able to add lots of light from separate flash units and even then the focus motor will probably be too slow to keep up with the action.
Canon's newer model 70-300 is much faster and the optics are good. You can often find this lens used as guys upgrade to the more expensive L line of Canon Lenses.
Now if you really want a performer start looking for deals on the 70-200 f2.8 version I. Much more expensive, you will pay roughly $1300-$1500 for this lens but the results are stunning and the lens will hold its value.