Non Chemical Cleaner for DD's Bath

I'm looking for suggestions on a "non chemical" way to clean DD's bath.. As I always fill to the same depth I am getting a bath ring and I'm wondering what is the safest way to clean this, making sure it's nice and clean but without having to use chemicals..

So far I've used a pine o clean bathroom spray (yes I know it's a chemical) and I've just rinsed the bath out about 4 times by filling it all the way up (a huge waste of water) but I just don't want to accidentally leave a trace of it in the bath and then her next bath be a "pine o clean bath".. Obviously not good for her and not good for her skin.

Any suggestions on what'll be a great cleaner and safe would be great...

i use an enjo cloth!!

I have a book called 'green clean' filled with all kinds of recipes, and it suggests using vinegar (apparently doesn't leave a film). They say 55ml to 4 litres of water, or put it directly on a sponge for harder to remove dirt.
Another tip is to rub the area to be cleaned with half a lemon dipped on borax, baking soda or salt, rinse well and dry with a soft cloth.
Don't know if they work, but worth a try

i use both suggestions, a micro fibre cloth. And sometimes i put some water in the wath, a splash of vinegar and clean the whole bathroom. ;)

I use a scourer - generally the "ring" is just soap scum & cleans off pretty easily. A mum in my Mother's group said she uses baby oil to remove soap scu. I haven't tried it myself, but she swears by it.