If you've never made a sweater for a baby and if you've never tried a top down sweater in a raglan style by far the easiest is the combination of both. And once you've done one raglan baby sweater you can pretty much mindlessly do many more if you're so inclined.
The sweater in the above photos was made using the pattern for Wilhelmina's Baby Sweater though I started with a 1x1 rib for a higher turtle neck and did a deep 1x1 rib on the sleeves and body. The pattern for the baby sweater is sized 9-12 months. Wilhelmina is now 3 and a half but wears clothes for a 5yo I didn't have to adjust the pattern much and therein lies the beauty of top down. I started with the same amount of stitches but kept knitting past where the pattern says to stop to separate the sleeves from the body, had her try it on and went from there.
This one I adjusted for chunky yarn (left overs from other projects...these sweaters don't take much) and a wide roll neck. I guessed for the amount of stitches to cast on and had her try it on after a few rounds. After that you can pretty much eyeball it. Kids clothes are a lot more forgiving in that way especially in a top down raglan style. I wouldn't necessarily want this boxy style for myself but it sure looks cute on the wee ones.
The head band was a neck warmer they used to wear when they were babies that is the same amount of stitches and the same stitch pattern cast on for the mock turtleneck of Wilhelmina's Baby Sweater.