We had similar, yet very different, circumstances at the end of '08. My husband was diagnosed with a brain tumor and req'd brain surgery. I have a 5 1/2 yr. old, 2 1/2 yr. old and 9 mo. old. We talked primarily with the oldest, just talking about what we did know on things he could understand...daddy needs surgery, the dr. will look in daddy's head, he will be in the hospital. We also said we were scared (which gave him permission to be scared), and that we didn't know the outcome (that was his one ? for us...will you be okay), but hopefully God would make us and him okay. We also talked to his teacher so that she could give him extra love (unfortunately, she gave him a lot of leeway and he became a small behavior problem, but he's back under control now) and send him to guidance if she felt necessary or if he asked to talk about it more than she felt comfortable.
I would definitely wait until you have a definitive plan of care, whether that's a surgery where you'll be out of the house for a couple of days or the biopsy results (for us, we had to with the surgery since there's no way to biopsy a brain tumor without the full surgery access). I would give a minimum of info. and remind my child that he/she can ask questions and it's okay to be afraid. I would try to keep things as normal as possible while you are undergoing any treatments without putting your health at risk.
Please feel free to pm if you'd like to talk. All of this is very scary. Also, remember to take care of yourself, and if need be see a counselor. There's nothing wrong with needing to talk!
M.