If they are extremely small, they could be fungus gnats. If they are fruit fly sized, there may be some hidden poop in her tub that you missed that they are breeding on.
If you don't want to change substrate to aspen, etc. you can make a fly trap outside her tub. I put a little water, dish soap, coke or other sweet drink, and maybe a little vinegar. This works well with fruit flies, I am not sure about fungus gnats but it's worth a try.
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