Help! With Amphibians!

[Malx]

Remember the peripheral pools at the local swimming pool when you were a kid?
You had the main big pool, which was deep, square and dull.
Then you had the other, more child-friendly ones set off to the sides.

The latter's what a bog-pool is. It's a safe environment for the lesser swimmer - animals which require water, but not too much of it.
Same as for human animals' offspring, which is why a bog-pool is completely safe for kids.

Regarding choice of soil, you don't want a light loam like peat. For once you'd be better off with a mix which is more clay and sand. Too much leaf litter-type detritus will just float away and decompose in the water, making it too nutrient rich and gunked up.