Baby sweaters. They go fast and everything you do is done on adult sweaters (to a point, see below).
I would put off top down in the round until you know techniques like short rows, provisional cast on, darts and plenty of math.
Without some short rowing across the back at the right place the collar rides too high in the front and too low in the back. You can short row around the collar to fix the back riding too low but if you don't do it across the shoulders it still rides high in the front (and you have to drop the front). suzeeq's third link has a page about it, but I think it would take some precise measuring to get a generator to get it right for you.
Those with bodily protrusions would need darts to match.
Trying to figure out where the arms and chest get separated if you have a stitch pattern can be a major trial.
Even a provisional doesn't do a perfect job of lining up the stitches on the collar, but it's the best I've found.
Sure you can try on a top down as you go but when it's wrong that means a lot of frogging because you won't really know it's wrong for quite a few rows. You don't find out the collar is wrong until much much later, it's like irregular jeans with twisted legs, feels OK in the store but once you start wearing them they drive you nuts.
Top down adults do go fast until you start frogging.
I ended up using a sewing machine and scissors to seam my first top down made to try on as I went so it would fit, and the collar still rides up in the front, which also required the fold over collar be stitched down.
Using that one as a pattern for my second one still required a lot of frogging.
The good news is young kids don't have much body shape so a generic top down doesn't take much shaping to fit right (or bad news if you figure a top down baby sweater would be good practice for an adult top down). It doesn't take much time at all to turn out top down baby sweaters.