Resources for flowchart programming for young, new programmers?
Honestly, I would be surprised if this method bore the kind of fruit that you hope for.  While flowcharts seem like a nice way to make ideas more concrete, I contend that they will actually become another level of abstraction that makes the task harder for middle-school students, who are only now making the transition from concrete to abstract thinking.
You're moving from boxes that represent ideas (Scratch, already one layer of abstraction) to boxes that represent arbitrary groups of boxes (now two layers, with fuzzy bottom-layer boundaries designated in the bottom later), and then making macro-decisions using those larger boxes (two layers plus fuzzy boundaries plus a lot of flexible thinking).
In short, I don't think you're going to find a big coding speed-up, or even higher quality output, from the students.  In this regard, it's a trap.
However!  I'm not going to suggest that kids can't use flowcharts.  In fact, I'm not even going to suggest that it's not valuable.  Kids who are beginning to develop abstract thought need to practice that skill-set in order to develop a richer ability to grasp abstractions as an adult.  However, I think that to do it well, you would have to be okay with the idea that it probably won't particularly help the kids with the actual coding tasks themselves, and furthermore decide that it is still worthwhile for other reasons.
If you still wanted to proceed, I would take this backwards approach:

Have the kids code in Scratch first, which gets them started with as little abstraction as possible.
Next have them construct flowcharts of what they have done, including a one-sentence description, and a NAME in the box (getting a good NAME can be quite hard)
At this point, they would be better able to manipulate those boxes.  You can ask them to present their flowchart structures (and the accompanying programs, of course!) to the rest of the class.  You can also use the diagrams that they have created to help them if they get stuck (though they may have trouble applying the solutions for all of the reasons listed above).

If you want to help them work in the other direction, you can provide a flowchart that works well, and then have them implement it in Scratch.  That can provide support for the concrete -&gt; abstract transition they're undergoing, and also help them learn to move in both directions (Scratch-&gt;flowchart, flowchart-&gt;Scratch).
I wouldn't ask them to work with poorly designed flowcharts (such as the ones that they themselves would make) from the start -- it's not clear to me that you'll get any sort of good result from an unsupported approach like this in a classroom setting.