Are there actually concrete risks to children, or is this a political pressure thing (if the vaccine hurts children, then it will blow up badly in the FDA's faces if they approve it, so they're being hesitant)?

That is, are the "risks" real scientifically-documented complications (either from a clinical trial, or from solid arguments about mRNA vaccines and children), or are the "risks" just unknown-unknowns and a lack of sufficient data?

I don't trust the FDA. I believe they're subject to a bunch of nonsense political pressures. I believe they employ a bunch of good people trying to do the best job they can within a US federal bureaucracy and all the confines of that (including the inability to compete on salary with the private sector for skilled/experienced candidates). I have no reason to believe they're more competent than, oh, the DoD. So my distrust of the FDA doesn't extend to believing that they approve the vaccine, it's because they really want to implant microchips in us, but it absolutely extends to believing that if they don't approve the vaccine, it may well be for reasons entirely unrelated to its scientific merit.