I need an adventure for my 40th, with the kids

Totally possible with those kids, and all the more fun for having them along. There are plenty of trekking operators in Kathmandu with solid reputations who could organise this trip at a decent price, but I wouldn't advise going with them. Nothing at all wrong with the way they operate but when you're doing a fairly hardcore adventure with children you need to be absolutely sure everything is going to go right and for that reason I'd be talking to World Expeditions worldexpeditions.com. WE has been introducing Aussies to the wonders of Nepal's Himalayan region since the 1970s and they can answer any questions and concerns you might have, tailor a trip suitable for your family, advise on all the necessary health precautions, arrange transfers from the moment you arrive in Nepal and make sure you're well-prepared mentally as well as physically.

I travelled with WE when we did a similar trip about 15 years ago with my daughters, aged about 6 and 11 at the time. We didn't go all the way to Annapurna Base Camp but it was a wonderful experience for all of us, and the girls handled the walking just fine. Prime time for trekking in Nepal is April-May and September-October. I generally favour the second option since the weather seems more stable than in spring. Bear in mind that the trek to Annapurna Base Camp is a lot more popular than it was when you were backpacking, and more people means you're never far from a tea shop and hot showers, but the scenery is as spectacular as ever.