In reply to James Raitt:
Having done a number of the classic Nepal treks with kids of that sort of age I would def agree about avoiding Annapurna Sanctuary - its busy and most of the days are through the trees so it lacks variety of terrain. Plus base camp itself is high, cold and busy so whilst its a spectacular place your kids are likely to not be in great form. If you want to go to Pokhara/the Annapurna region then take a look at the Mardi Himal trek which runs next door to the Sanctuary trek (its not that difficult to combine the two over the course of 7 or 8 days trekking but perhaps ambitious for a first trip).
I'd suggest you consider either the Tamang Heritage Trail (easiest and with some proper culture but few really good views and a pretty long drive in and out), the Langtang valley, or Gosainkund/Lauribina Pass. They are all in the same area - north of Kathmandu - and benefit from relatively easy access by road, less crowds, decent tea houses, good proper himalayan mountain views but without being too high and cold. For my money the Lantang valley is the best.
Robin Boustead's book on the Great Himalaya Trail is geared towards big blokes with unruly beards and huge rucksacks rather than families with kids but I'd recommend it, you'll find it very helpful.
If you are with kids, hire a jeep to get you there and back so that you can keep the driver sensible. The shared jeeps and busses are cheap but the drivers can be young and crazy.
Lots to consider such as whether to get a guide. Drop me a line and I can happily bore you to death on all this.