In the Squamish Valley, the sun goes behind the mountains around 2pm and a low fog often sets in, it gets very cold dressed up like a Simms rep with several layers of wool and fleece. Its several very noticeable degrees colder than Squamish proper, and more humidity. The Squamish River changes every high water and that puts a lot of wood in the river, so if they were to go into the (ice cold) water, they're in pretty bad shape. You're out of cell phone range up there and there isn't much traffic- a couple of anglers or hunters and some powerhouse workers on weekdays. The road is unplowed beyond the powerhouse and requires a high-clearance vehicle with 4wd if you want to turn around. I've had to pull several people out up there and I've encountered more who at lost headed in the wrong direction, way under equipped and would have been in for a very long uncomfortable night. Its easy to forget how remote it really is up there and how little help there is if things went sideways.
In your case, you want to fish and to catch fish on the Squamish, you have to cover a LOT of water which means a LOT of walking and climbing over log jams, more so this year because the river cut a new channel thats is mostly pretty removed from the road this year. Not very condusive to bringing kids. I wouldn't bring kids to the upper in winter, at least not without another 4wd truck (not an AWD soccer mom car) and I'd abandon the idea of wetting a line if you have the kids with you.
If it were me, I'd check out the lower Squamish by the dike or even along bar at the mouth of the Mamquam and watch the eagles, there are more around this year than last year. Chum are on their redds now, spawning, neat to watch, but their no longer suitable for angling. THey do bring in the eagles though.