As a family route, i would hesitate to recommend Cwm Glas, as i am unsure of the capabilities of family members when it comes to mild scrambling on the ascent of Crib Y Ddysgyl.
Navigating in Cwm Glas is very straightforward, as once you enter the inner slopes of the Cwm, there are several very well used and clearly defined paths, that will take you to the upper reaches below the Northern cliffs of Crib Goch.
It sounds very intimidating, and from the car park in the llanberis Path below Pont Y Cromlech, it looks very intimidating.
Once you have left the main road, the paths into the Cwm show themselves clearly, and navigating to the foot of Crib Goch is very straight forward, mostly on steep grass and bouldery rock, but very safe and with no sheer drops, if care is taken, its straight forward and there are several alternative routes, all straight forward to navigate.
Yes there are many challenging scrambling routes inside the Cwm, the Parsons Nose on llechog being one of the best, but keep to the well defined paths, and the Cwm will be a revelation to a first time visitor.
There is a little bit of moderate climbing, but its mostly on grass and its within the capabilities of any fit family group.
The access to the scree leading off Bwlch Coch into Cwm Glas and to the saddle between Crib Goch and Crib Y Ddysgyl, is straight forward, the route and direction is never in doubt, in clear weather, and obviously if the mist is down, the whole Cwm needs avoiding.
There is only one small technical bit of scrambling, and that is to gain the path on towards Crib Y ddysgyl, as if you were doing the Snowdon Horseshoe, but its very short, and is taken by thousands of walkers annually.
I would recommend you first doing a traverse of Cwm Glas by yourself, before taking a family group, and then you could make the decision for yourself, but Cwm Glas is very safe and straight forward, as long as one does not go adventuring on to dangerous ground.
That dangerous ground is obvious, and it can all be avoided, so Cwm Glas has something for everyone.
Stay within the Cwm, on on the clearly defined paths which are there to be followed,, and i would recommend the route to Snowdon for any keen fit party.
Cwm Glas looks mighty intimidating from the road, take my word for it, its safe and offers a very rewarding alternative up Snowdon.
If you want solitude when the ridges above you are swarming with visitors, Cwm glas comes highly recommended, but if care is taken, the Cwm is safe and rewarding.