I'm a little puzzled...
The title of the thread is alternative to 18-135 for kid candids, but the body of your post is looking for a walk-around.
There's nothing wrong with the 18-135 for kid candids...especially outdoors in daylight. Where that lens comes up short is indoors as it's a slow f4.5-5.6, and reaches f5.6 about 70mm or so. What it definitely needs indoors is an external flash...perhaps a 440EX or a 580EX i or ii. Bounce the flash off the ceiling and you should have good results. That's how I solved the problems with my 18-135 on the 30D and later 60D until I upgraded to L glass.
If you're really looking for a fast 'indoor' lens, how about the 50 mm 1.8, 1.4, or the pancake 40mm?
I still think the 18-135 is a near-perfect walk-around lens, even more so than the 24-105L I replaced it with because of it's great zoom range with surprisingly good IQ.