@Asyas_illos very cute! Love the colors, the reading, the engaging characters, and how you've worked in SF landmarks.
Here's something to think about: what is your focal point in this illustration? Do you want the landscape to dominate (which it's doing now), the focus on SF, or on the kids reading?
I took a screenshot and circled the areas where things are happening -- notice all that "dead" space? Instead of what's basically a cute landscape as a banner, why not showcase the kids, the heart of why SCBWI does what it does?
Here's a drawover, just a suggestion of what you could do to increase the impact of this sweet banner:
Here are suggestions based on the drawover:
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Increase the size of the characters, giving them more visual weight.
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Balance out the composition -- think about adding a kid or two running to join the group -- adds some life and interest to a static scene.
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Switch the direction of the plane, so it's flying toward the action, leading the eye to the characters and text. (Also consider increasing its size so that "go read" is legible.
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Play with your values even more. Darken the seagulls so they're more visible. You may also want to play with the values on the characters so that they stand out more from the midtown background.
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Consider shrinking the Golden Gate Bridge so that it's more in the background -- it will still be there, but the focus will be squarely on the kids and banner text.
Of course, these are only suggestions. This is YOUR art and you get to do what you want with it! 
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I’ve got about another week before entries are due for a new regional masthead for SCBWI here in northern ca, and I’d like some feedback on what I’ve got so far in terms of composition and color choices. The black birds are place holders for where’d I’d like to put some seagulls. The characters and cargo ship are on a separate layer, so are easily moved. Please let me know your thoughts and ideas on how this is turning out! Thanks!