A Roadside Stand describes a small-time farmer trying to sell their produce from a stall by a busy road. The farmer is poor, wanting only a small slice of city wealth, and feels bitter that drivers won't even look at the stand, let alone stop and buy something.
In this context the "trusting sorrow" is a neat encapsulation to express what is described over following few lines: that modern life offers an unspoken promise, which is not being kept.
Here far from the city we make our roadside stand
And ask for some city money to feel in hand
This expresses the desire of poor country folk to share in just a little of the city's wealth.
And give us the life of the moving-pictures’ promise
That the party in power is said to be keeping from us
This is a reference to media and politics. The "moving-pictures’ promise" is that on-screen, people do well if they work hard. It's not just cinema but a representation of the "American dream", which finds perhaps its clearest expression in Hollywood.
The farmer is working hard, not wanting to fall back on charity or the state, but is not being rewarded. This is alluded to later in the poem:
Swarm over their lives enforcing benefits
That are calculated to soothe them out of their wits,
And by teaching them how to sleep they sleep all day,
Destroy their sleeping at night the ancient way.
For those that feel the pinch of rural poverty in spite of hard work often blame their plight on the metropolitan establishment, politicians who favour the cities where they live over the more sparsely populated countryside. Hence it's the "party in power" whose policies are stopping wealth from flowing out of the city into the country.
Thus "trusting sorrow" is a phrase to briefly express that the farmer "trusts" the unspoken "moving-pictures’ promise" and is "sorrowful" that politicians have been "keeping from us" the wealth of the city.