Whats a good lense for a Rebel XT

I need a lense for a Rebel XT to take pictures mostly of my son indoors.

Is the 18-55 that comes with most Rebel XT a good one to get?
How much do they run?

I noticed some retailers (Cannon Rebel XT) include a lense (18-55) and some don't.
Is it better to get the lense included from a money stand point?

Thanks

The kit lens(18-55)is merely adequate, and really needs to be stopped down to acheive good sharpness, so indoors it's not really useful.
The 50 f1.8 or 50 f/1.4 are very sharp and have the speed you will need indoors, the downside is that with the crop factor they have the FOV of an 80mm lens.
The 35mm f/2 and 28 f/2.8 offer a wider FOV and decent although not blazing speed.
Zoom lenses tend to be slower than primes unless you want to jump into the L series of lenses. The L lenses are very expensive, though. I really don't know a lot about the third party lenses out there although I heard very good things about the Tamron 28-75 f/2.8 xr di.
All that being said, a flash is really needed to get decently lit photos indooors, the pop up flash will work but will produce the deer in the headlights look a fair amount. A speed light or equivelent the will bounce will provide more natural light.

King- Consider a few things first. Assuming your son is young and not walking yet - he's inside most of the time. Fine for now. Is he going to be playing outdoors this fall?
A BIG mistake many people make is buying gear that is 'adequate' for what they think will satisfy their immediate needs.
Aside from your son expanding his horizons - what happens if you grow in your photography?
A comparetively 'cheap' kit lens will quickly end up in the back of your closet and has no resale value other than a reminder of your mistake.
The XT is not a camera to underestimate as to capabilities. Put a decent quality, decent range, reasonably fast lens on it and then have REAL enjoyment.
What us old timers know is that the lens is as (if not more) important than the box you mount it to.
If you want to mess around with a "general" decent lens - buy the Canon 50mm f1.4 - they are a good all around beginning lens. $75.00 or so. surprisingly sharp !

The 50 f/1.4 is around $300, while the 50 f/1.8 is around $75. The 1.4 does have a metal mount and is a half stop faster than the 1.8. Either lens would be a great lens to start with, although a little limited indoors by their equivelent focal length of 80mm(just a little long).
setprime is totally correct though when he says the lens is as imortant or more so than the "box"(body) it is attached to, I would say more imortant in every way though. A great lens on a Dreb will blow away a poor lens on a 1D. Garbage in = garbage out, and a photo taken with a poor lens no matter what body will result in a comparitively poor photo.

I can't say enough good things about my EF-S 17-85 IS USM.

28-135 is a good al around lens. Get it with a hood and a UV filter and you'll use it alot.