The NW ridge is steep and unremitting I took our children up it when they were aged 4-7. They got to the top with some carrying from me, but they weren't at all happy! Your children may well be used to steep slogs, but it might be worth considering doing the walk the other way round.
It's the normal descent from climbs on Cwm Silyn. I haven't been down it for a few years, but never had any problem descending it in rock shoes. My wife, a non climber, managed it when she was about 7 months pregnant. It's a bit loose in places, but much easier in ascent than descent.
However, there has recently been a big rockfall at Cwm Silyn, which has effected several of the climbing routes. It doesn't sound as though it's affected the descent route.
You can park at the end of the lane that leads up to Llyn Silyn and return down the NW slope from Carnedd Goch, if you want to make the walk a bit shorter. It's an easy grassy slope. I once descended it in Koflachs using crutches when I went climbing on Great Slab while recovering from a broken heel.
Apparently mobile phone coverage is good for almost all the Everest Base Camp route. Don't know about Annapurna. However, I would expect leaders of commercial treks to carry Sat Phones, which should work almost anywhere. Hopefully, you will hear something soon.