Not sure about starting CO for baby sweater sleeve.

Working boy’s right front. Have knitted 5 1/2". End with RS row. For both versions:
Next row WS CO 24 sts, work 5 sts in seed st , p to last 5 sts, work in seed st, 48 sts total.
I am at the end of the RS but don’t understand with the 24 CO how do I "work 5 sts in seed st, p to last 5 sts? I will use the thumb CO.

Cast on the 24sts then start working over those same cast on sts. Seed stitch 5sts at the cuff and then purl across to the last 5sts and seed stitch those 5.
The thumb cast on isn’t the ideal cast on for this many sts. The knit or cable cast on is better. Both are demonstrated at the Free Videos tab at the top of the page, Cast Ons.
What pattern are you using? It’ll help to have even the name so that we can see what it looks like.