Need help to enlarge a crochet sweater pattern for child

I have this adorable crochet pattern for a baby 6-9 months old. I want to be able to make it sizes newborn, 12 mo, 18 mo, and 24 mo sizes. Can anyone help me figure out how to change it? I have only been crocheting for about 2 years and am not all that great at knowing how to increase/decrease. Please help!!

Sport weight yarn (Jamie by Lionbrand was used in the models shown)
"J" hook
Size: 20" chest (3-9 months)
Gauge(on yoke): 1 inc and 2 hdc= 1"(measured at the base of the stitches)
*BEG chain does NOT count as first st!
Ch 32
1. Sc in second ch from hook and in each ch across. (31 sc). Ch 2, turn
2. Hdc in first 2 sts, 2hdc in next st (inc made). *hdc in next 2 sts. Inc in next. Rep * across to last st. hdc in last st. Ch 2, turn.
3. Hdc in next 2 sts. Inc, *hdc in next 3 sts, inc. Rep * across. hdc in last 2 sts. Ch 2, turn.
4-7. Continue working hdc in each st across with inc in 2nd hdc of inc in row below. Ch 2, turn.
8. Hdc in 1st 5 sts. Inc. Hdc in next 8 sts. Inc. ch 5 (for armhole). Sk 16 sts, hdc in next st, inc in next. hdc in next 8 sts. *Inc. hdc in next 8 sts. Rep * once more. Inc. Ch 5, sk 16 sts. Hdc in next st. Inc. Hdc in next 8 sts. Inc. Hdc in last 4 sts. Ch 3, turn. Do not fasten off.

Body: Beg chain counts as a stitch on these rows
Stitches: V-st[(Dc, ch 1, Dc) in space indicated] Cluster[3dc in same sp]
9. Dc in 1st st. Sk 2 sts. *V-st in next, sk 2 sts. Rep * across to last 2 sts. Dc in last st. Ch 3, turn
10.Dc in 1st st. Cluster in each ch1-sp of v-sts. Dc in beg st. Ch 3 turn
Rep 9 & 10 for body pattern to a total of 16 rows from beginning. Do not fasten off.

Stitches: Picot[ch 3, sl st into last st made]
Sl st up front in ends of rows, adding 3 evenly spaced button holes (ch 1 or ch 2 instead of sts) between rows 1-8. Sc around neck, adding picots if desired(for lacier look). Sc down other front. Sc across bottom placing picots every 3rd st. Fasten off.

Attatch yarn to underarm. Place between 20-23 sts around. Join. Ch 2, without turning, countinue to work rnds of hdc until legnth of sleeve is desired.(about 10 rows).
11. Hdc-dec in every 3rd st.
12. Sl st around loosely, placing a picot every 3 sts. Fasten off.
Repeat for other sleeve