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A common model for chemical intake is based on the person's weight. I.e. if it's unsafe for the average human it's likely even worse for children, smaller animals, etc.

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This would need to account for how much more or less of the chemical that the person consumes (e.g. children would likely drink proportionately less water all things considered)

Data point = 1 but my kids eat/drink 2-3x less than I do. They weigh 5-7x less.

That's because they're growing, not because they weigh less.

In general, smaller organisms tend to consume more calories or resources per mass of body weight than larger ones. One reason, for mammals especially, is that maintaining homeostasis means scaling energy input with surface area, since that’s what radiates away heat. Surface area roughly scales with square of height, while mass scales with volume (cube of height).

> children would likely drink proportionately less water all things considered

Children tend to be more physically active, and have higher surface area to volume ratios, so proportional to body mass their water intake must be higher.

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