For kids indoors, there are several ways, I would start in av mode, ai servo, single point focus, and play with your iso setting until your shutter speed is around 1/400....to start anyway....
Here are a few 18-135 for sale here for around 250 new
I'm going to suggest that a slow zoom lens (f/5.6) used indoors witthout flash isn't realistic for most situations. That's why I suggest the OP get a Speedlite first.
F/5.6, 1/400 and indoors as you suggest is going to need ISO 12800 or more most of the time.