I am making a baby sweater from a pattern with no picture. It says
"Pick up sleeve stitches on holder on dp needles. Join yarn at underarm and work to desired length."

What is the desired length? I haven't been around babies in centuries! and have no idea how long their arms are. The pattern says that it is for a 4 to 10 month old although it looks a little small for a 10 mon. Maybe a 6 mon old. Anyone with a baby that age that can measure the length of their arms for me? :??

Also, what do I use to sew the buttons on? I don't want any swallowed!

I just measured my 3 1/2 month old: About 6 inches/15 cm from armpit to wrist. She's pretty tall for her age.

I've just used the same yarn for sewing on buttons on baby sweaters and have never had one fall off. Just make sure to sew them on tightly. Another thing regarding buttons: Even though those buttons that come in various forms and shapes are incredibly cute, they can be such a pain to use. I made a cardigan for my oldest before she was born with very cute clown-shaped buttons. Of course, it took ages to button and unbutton it, so I ended up just keeping it buttoned and using it as a pullover.

LOL! I never thought about the cute buttons being hard to use. Thank you for the information. I'll go start on those arms now!

I'd recommend a zipper instead of buttons. Buttons can also be a choking hazard, even if they're just there for decoration. Velcro could be an alternative way. We discovered, a bit late, that our son loved chewing on velcro when he was teething too :D

If you do sew buttons on, be sure to attach it to more than one strand of yarn, because that strand can be broken if the button is pulled. You can also line the inside with something to stabilize the button, like a firm tape along the inside of the button band.