Tadpole development
 

I am trying to rear tadpoles with my children but they keep dying just as they get interesting. When their front legs start to develop and when they can climb out of the water onto carefully positioned rocks, they usually die a day or two later.
We are keeping them on the patio in a cat litter tray (clean, obviously), with water from our neighbour's pond where they first came from. There are rocks in there for them and pond weed. We feen them flake fish food every day and try to change the water when it looks particularly yukky.
Do they need oxygenating plants in with them? Is the water too dirty? It looks ok, with dirt on the bottom.
The children are getting quite distressed as this is our 4th attempt and they just keep dying on us.
Any advice would be much appreciated.