You haven't said how old your children are. We walked up (and down) Snowdon with DS1 and DS2 last year, aged 12 and 8. So guessing they were probably older than yours are now? However saw plenty (in fact crowds) of people walking along the route we took (Pyg track) and you do get lovely views (moutains, lakes below etc)even early on the route. So you could walk some of the route, and then come down again.
The downside of getting to the top, whether by foot or by rail, is the weather. Was gorgeous when we set off (blue sky, cloudless etc) but when we were within half an hour of the summit, cloud came down and we couldn't see a thing from the top. Could see the train, but could not see the passengers on it, it was that bad. However we cheered ourselves up with the knowledge that we had walked up for free, those on the train had paid £25 each, and had the same (poor) view as us.
Another point about the train is it gets very booked up very quickly. So if we had wanted to use it, could only have travelled on first train of day, I think.
We set off on on our walk from Pen-y-Pass, there is a car park there but was full by the time we got there (around 10am) and someone told us it is usually full by 9am on a good day. Instead we used the park and ride from Llanberis (I think). Cost the 4 of us almost £10 but in my opinion £10 well spent.
Sorry has turned into a bit of an essay. Apologies if I have any of the facts wrong, writing off the top of my head!
V envious as loved the scenery in Snowdonia. Recommend Conwy Castle & Bodnant Gardens also, will rack my brains for other things. Oh yes Greenwood Forest Park was great (google it) though recommend you arrive early.