First, Rick is correct--get the rest of your gear first. The pack has to fit you, fit your gear and be comfortable for you with the gear in it.

Several of the "cottage" companies are making gear for extra big and tall folks, who obviously can't use the "one size fits all" used by a number of mainstream companies and REI's house brands. Being short and stout myself, I'm at the other end of the spectrum--many packs aren't made in a short enough torso length. So I can sympathize!

There is a blog by a well known ultralight hiker, "Jolly Green Giant," who does keep a close eye the big and tall market--he's 6'5" and 280 lbs. Here is his recent entry about the 2012 Gossamer Gear Mariposa now being made in extra-large for tall folks.

A number of the "cottage" companies will also customize packs to fit. Extra charge, of course.

EDIT, LATER: Do check the manufacturers' websites for the maximum recommended load. From my own experience with both children and grandchildren, "UL" and sherpa-ing for young kids don't go together 100%. For one thing, each kid needs a complete change of clothes--if there's any water around, they'll get wet! You'll need a large capacity pack and even with lightweight gear will probably be carrying 35-40 pounds. At 4-5 years old, the kids won't be able to carry more than a pint water bottle, a jacket, rain gear and snacks.