Which bottom feeder??

a group of cory cats

If you're looking to reduce algae a bristlenose pleco, or some otocinclus would help.

But if she's having algae problems you should really look at her water parameters and see if excess nutrients are leading to algae growth, and also limit the time the lights are on (about 7-8 hrs is sufficient).
A 40 liter tank is not that big and nutrients will build up quickly leading to excess algae growth. Adding more fish will not help with that and would probably make it worse.

You said she's little, so she may also be putting too much food in, which would raise nitrates and phosphates and lead to algae.

You may also want to cut down the amount of time that the lights are on in the tank. Long periods of light can lead to excess algae growth. Cory cats will not, to my knowledge, eat algae. A brislenose pleco will stay small (4-5"), and do a great job on algae, as will a small group of otocinclus. A little harder to come by, sewellia lineolata loaches, will also do a great job on the algae, and they only get to be about an inch or so long. Good luck.

I have a beautiful little albino bristlenose pleco. He isnt at all scary looking and even my 4 yr old loves him whereas she doesnt at all like the browner or blacker colored ones. Maybe try to find one of those?

the only reason for adding a fish to the tank is if the tank owner enjoys the fish, not because the fish 'does its job'.

40 litres = 10 gallons, so you have to be very careful about keeping the water stable in a small tank like that. perhaps watch her feeding the fish one day to see if she is giving them too much, as that could easily be the source of the problem.

what is the water change schedule and how much each time?

Ideally she would have frequent water changes, proper feeding of the fish, and a separate brackish tank for the molly.

from what i have read mollies are fine in freshwater for their whole lives.

Maybe a mystery or a nerite snail