IMO, the Fraser River sockeye salmon fishery is not suitable for a 6 year old to participate in, regardless how mobile he or she is. The swift current of the river, the shore accessibility and the amount of fishing pressure can all make this excursion not very enjoyable.
If salmon is the target, and you are not too selective on the species, then my suggestion is to wait a couple of months until early October when many better opportunities open up for this. Here are my suggestions.
The Vedder Canal, once chinook, coho and chum salmon start making their way in around the end of September, can be very good for this. Beside the slight climb up and down the dyke, the beach is sandy, smooth and flat where she can either stand or sit on to fish from. The depth change is gradual so she will not be in any danger. Find a tailout of a run where coho salmon love to hold and she can do very well by casting and retrieving a spoon.
Here is a video Chris shot of me fishing there a few years ago and you can see the sandy beach where she should be able to fish from:
https://www.youtube.com/watch?v=sNCQJaO-gUgThe second option, which I like the most, is to fish for coho salmon
on the bars at Glen Valley Regional Park once it is open on October 4th. The bar is primarily made of small gravel rocks and the bank is fairly flat so there's no immediate danger. You can either bottom fish with roe, or again, cast and retrieve a lure.
Here is a video we did there last year and you can see the access:
https://www.youtube.com/watch?v=d6FbfDjUehkBTW, the shoreline in front of Peach Road on the Vedder is not suitable for her. It's too steep and the water is quite deep directly in front of it.
Good luck!