No, there's just no way Prue could have had a kid and Piper not know about it; they were just to close for that - if nothing else someone would have figured it out in that family...they never really were that great at keeping secrets from each other - except for maybe Phoebe when Cole was the topic. Also, Prue just never seemed to get over lossing Andy enough to move on with some other guy to the point she'd start a family with him...Andy really was it for her and without him she just didn't seem like she'd ever get married or find the right guy again; plus, even after him she was too busy to have one - what with all the demon hunting and taking care of her sisters still and her job as a photographer...she just didn't have the time.
But mostly I don't think she could have had a secret kid because like I said - there's no way Piper wouldn't have found out about it and if they'd killed Prue like they did...you wouldn't even get lose to that kid with Piper around, she'd be so attached to it because it's a part of Prue - no way could the eldest Halliwell have a child and the middle on not know about it; I just don't buy it.
Yes, I think Prue would have made a wonderful mother if she'd become one, a very protective mother too, but no....I just don't see her with a secret child out there. Sorry.