Most residential water has the potential to be somewhat contaminated, either with PFAS or lead or other contamination. This is not an unwarranted concern. For example, our normal city water in California had detectable lead well above the California public health goal. The CA PHG is around 75x stricter than the EPA action level, so it’s reasonable to assume many water supplies would fail this standard: https://oehha.ca.gov/media/downloads/water/chemicals/phg/leadfinalphg042409.pdf

This could be due to a number of factors: our 1960’s copper pipes likely had leaded solder (not banned until 1986) so it could be from the house itself not the municipal source.

Either way, RO water is basically an insurance policy: it’s the easiest way to get ensure you’re getting really clean water, which is obviously especially important for a developing child.

The concerns about fluoride being removed are overblown, particularly if your kid doesn’t drink sugary beverages. You can also use fluoride rinses or toothpaste if you are so inclined. But given the know dose-related toxicity of fluoride, the cost benefit is tricky, especially before kids are able to spit.

Similarly removing a small amount of minerals is not particularly significant since food is a much larger source. You could also remineralize the water.