You'll enjoy the 7D. The 18-135 isn't the best lens around but it's tremendously versatile and you'll find there are many times when you don't know exactly what range you need before the shoot, so versatility is a good thing. You can surely sell it for what it cost you in the kit.
Starting with your most urgent need, the baptism. It sounds like it's not your church since you're uncertain about the light. Can you ask someone if flash is OK during the sacrament in this church? If so you might want to invest in a flashgun such as a Canon 430 EXII and a flash diffuser. [You'll want these for pictures of your kid also.] With a flash (even the pop up flash) use "P" mode and ISO 800.
If you cannot or do not want to use flash then you'll need to bump up the ISO. I'd set the ISO to "auto" and shoot in Tv (shutter speed priority) and set the shutter speed to about 1/200 sec. In auto mode the ISO will adjust up to 3200. Make sure that your aperture display isn't blinking, meaning you need a slower shutter. [You don't need to increase your shutter speed when you increase ISO, by the way. It's the other way around. Increasing your shutter speed reduces the amount of light going to the sensor and therefore requires you to raise the ISO, all else equal.]
Your biggest problem will likely be white balance. I strongly suggest shooting in raw so that you can adjust this later. If you shoot in jpeg you'll probably need to set a custom white balance and this is probably not the time to learn that skill.
If your sports shooting is all going to be outdoors and in daylight then you don't really need a "fast" (f2.8 or f4) lens. I'd consider the
new Tamron 70-300 VC for $399. It hasn't been released in Canon mount yet but you can preorder and take advantage of the rebate.
Let us know how much you are willing to spend on lenses before we start recommending expensive "L" glass.