It's not a stupid question, and I've heard that some vaccines are aiming to be approved only for adults at first. This would make sense not only because, as you

It's not a stupid question, and I've heard that some vaccines are aiming to be approved only for adults at first. This would make sense not only because, as you say, you can start to approach herd immunity without vaccinating everyone, but also because children are weaker spreaders. Obviously "herd immunity" is more of a gradient than a cut-off, and it's much easier to approach it if we can vaccinate everyone, but we can definitely speed up the process (including the months-long process to even get the vaccines to everyone) if we can start vaccinating some people early.