I agree with Faith. Make sure that you are in a good place (dealing with the pregnancy) before sharing with your other children. I have a 9 year old, so I know how perceptive they can be. They will pick-up on your attitude and carry it forward.
I'm not sure if you are religious or how you have dealt with the issue of babies, but I always told my children that a married couple prayed for a baby and if they were lucky, God blessed them with one. In my opinion, it's not the whole story, but something I won't have to recant later. My 9 year old is picking up on more that we are willing to admit, so I'm not sure if you can go the same route? A relative was in a similar situation as yours (unplanned pregnancy with her boyfriend). I told my children that sometimes God also surprises some couples. They took it very well and were happy for her.
I'm sure that your children will be excited to tell their classmates that they are going to be a big brother or big sister. Plus, this time around, you'll have 3 helpers and in a few years... babysitters :)
Good luck, and let us know how it goes.