Finished Measurements:
First size is from 5 to 9 months, second size in parentheses is from 1 year to 18 months.
Chest: 20 (22)" Length: 10 (12)"

Gauge: 7 sts/inch on size 4 needles (28 sts/4 inches) or size to get gauge. Entire sweater is worked on these needles.

Abbreviations
K, knit; P, purl; st(s), stitch(es); Rev St st, reverse stockinette stitch; rep, repeat; beg, begin(ning); tog, together; inc, increase; dec, decrease; rem, remain(ing); RS, right side; cont, continue; meas, measures; patt, pattern; WS, wrong side; YO, yarn over needle

Eyelet rib pattern (worked over 4 sts)
Rows 1 and 3: (wrong side): K2, p2
Row 2: P2, YO, ssk
Row 4: P2, k2 tog, YO Repeat these 4 rows for pattern.
Note: You will be making holes with YOs. You have just purled. If next st is to be knit, keep yarn in front and lay yarn over needle. If it is to be purled, wrap it over top of needle and bring it to front. You have just knit. If next st is to be knit, bring yarn to front and lay it over needle. If it is to be purled, bring yarn to front and wrap it over top of needle and back to front.

Back: CO 78 (86) sts and set up pattern on WS row as follows:
Rows 1 and 3: K2 (6) p2; * k7, p2, rep from * to last 2 (6) sts and knit them.
Row 2: P2 (6), YO, ssk; * p7, YO, ssk; rep from * to last 2 (6) sts and purl them.
Row 4: P2 (6), k2 tog, YO; * p7, k2 tog, YO, rep from * to last 2 (6) sts and purl them. [edited 11/10/10]
Cont in this manner for 3 (4)" and have completed a RS row. Work decs as follows: K2 (6); *p2, k2, k2 tog, k3; rep from * to last 2 (6) sts and knit them. [70 (78) sts]
Cont in pattern as established until work meas 6 (7)" from beginning and a RS row is completed. At this point change to working all sts in eyelet rib pattern and work an additional 4 (5)". BO all sts fairly loosely.

[edits made October 2012]
Right front: CO 42 (46) sts and set up pattern on WS row as follows:
Rows 1 and 3 (WS): K2 (6), p2; * k7, p2; rep from * to last 2 sts and knit them.
Row 2(RS): K2, YO, ssk; * p7, YO, ssk; rep from * to last 2 (6) sts and purl them.
Row 4(RS): K2, K2 tog, YO; * p7, k2 tog, YO; rep from * to the last 2 (6) sts and purl them.
Cont in this manner for 3 (4)” and dec in the k7 as on the back.  [38 (42) sts]. At 6 (7)” from beginning, change to working all sts in eyelet rib pattern as for back.
When yoke meas 2.5 (3.5)” and beginning on row 2 or 4, BO 12 sts in pattern and finish row.
Dec 1 st at (neck) edge every other row 4 times. When front meas same as back, BO all sts.

Left front: CO 42 (46) sts and set up pattern on a WS row as follows:
Rows 1 and 3: k2, p2; * k7, p2; rep from * to the last 2 (6) sts and knit them.
Row 2: P2 (6), YO, ssk; * p7, YO, ssk; rep from * to the last 2 sts and knit them.
Row 4: P2 (6), k2 tog, yo; * p7 k2 tog, YO; rep from * to the last 2 sts and knit them.
Work same as left front except BO for neck beginning on Row 1 or 3 and work decs at the same edge.

Sleeve: Sew sleeve seams. With RS facing, PU and knit 54 (66) sts along yoke edge - 27 (33) sts on either side of shoulder seam.
Set up pattern on a WS row as follows: K2, p2 * K6, p2; rep from * to the last 2 sts and knit them.
Work as on back with Rev St st and eyelet ribs. Keeping in pattern, dec 1 st at each end every 4th row 8 (10) times.
When sleeve meas 3.5 (5)" or 2" less than desired length to wrist, change to all eyelet rib pattern and work for 2". BO very loosely.

Finishing: With RS facing, PU and knit 60 sts evenly spaced around neck. Knit 2 rows and BO all sts. Buttonholes are easy. You’ve already made them with the eyelet rib. Determine where you want to place buttons opposite eyelet holes.