I have few memories of this movie, but I remember loving it so I am trying to find it for my children. The movie starts as a live action movie. It's older, kind of like Mary Poppins, and I think the children are English as well. I remember a pond in a large field outside of a large mansion. Somehow the children get into water and turn into cartoon fish. There is a kingdom in the water where they get into trouble with the ruler and are imprisoned. Somehow they escape and are turned back into human "live action" children again. That's all I can remember. I hope someone else remembers more and can help me. Thank you!

Could be The Waterbabies (http://www.imdb.com/title/tt0078477/).

Wow, that sounds like an awesome movie! If you want your children to watch a good cartoon movie that is also educational... I would suggest Disney's The Sword in the Stone. It's a great movie. I am 22 and I still watch it all the time.

Yes, I do think The Water Babies is the movie. Thank you so much! I'll have to see if my library has a copy.