Ash and Misty have never kissed, got married or laid a single hand on each other. There's no physical way they could have a daughter. But, there is a video on internet (I've seen it myself) showing Misty, Pikachu, Miranda (the blue haired woman from Mewtwo Strikes Back) and a pink haired little girl. Misty looks as young as she always did, she doesn't look like she would be a mom at that age. And I really doubt black and orange hair could make pink. They say the video was made to promote the first movie, but then they regretted it and never used it. Who and how did they manage to get this video? Who is this little girl? Why is Miranda in the video and not Ash? Welcome to the world of the unknown, I'm as clueless as you are. I'll try to see if I can upload the video on the page for you all to see it, and I'm not a Pokémon music expert, so if anyone can tell me which song is used I'd be thankful.
