I'm sure i must have let mine grow too old before picking. I found it very stringy and fibrous. With that said I would suggest harvesting young and often.

If there is fresh growth there is a chance it could be damaged. Like ruthieg says put a cloth type material over it for the night. you may need to put some sticks into the soil near the plants to keep the fabric from damaging the plants. If your plants are damaged you can re-seed either indoors or out. Raab sprouts and grows very quickly!

Mindy - I must relate my experiences with rapini. I was really getting into sauted kale and thought I'd try this rapini. I sauted in olive oil, then after about 2 minutes, I added a mix of soy sauce, a bit of liquid smoke, and some sanJ schezwan sauce - same as I do for most greens, added just a tad of water and cover to let it steam through for a minute or two. I couldn't believe how strong tasting it was, and I drizzled just a bit over rice with the juices to flavor the rice. I truly thought to myself that I would never buy that again, I was going back to kale and spinach. It must have been that I bought 2 bunches, because I had to have had it again, and I got to craving it so much it is unbelievable. I only have kale when I can't have rapini, and I have to have rapini 1-2 times per week minimum, and I make an absolute huge quantity~ about a pound, all for myself. I saute as above, serve over rice, and have a side of butter beans. That is my favorite meal. In fact - I think I'll have to go to the store and buy it tonight. I used up all my rapini in the garden 2 days ago... But seriously I went from putting maybe 1-2 spoonfuls onto a large serving of rice, to eating about 2/3 rapini to 1/3 rice. And I tell you - I just can't enough. There is some liver reaction to bitter, that my body must need or something. So after not liking the bitter, and my liver/gall blader going woopee about it, they must have told my mouth to love it, is all I can think. But I tell you now - it is my favorite food, hands down.

Those are the cotyledons. What those seedlings need now is stronger light. Starting a few more seeds than you need is commonly done. I'd pinch off the larger, spindly ones and save the newly-germinated ones, getting them into more light will give them a better start. It is, however, late for broccolis in your zone. You'll probably have better luck with your squash.