Following is an in-depth look at how I did it and everything you need to know to make one, too.

Things You Should Know

• You must have a copy of the pattern to follow along.  The Baby Surprise Jacket is available in The Opinionated Knitter, Knitting Workshop, a couple of back issues of Knitter’s and as a $3 single pattern leaflet from Schoolhouse Press.  You have no reason not to own your own copy. [Ravelry]
• It would be very helpful if you have already knit a regular BSJ without modifications.
• I have a hunch that two- and three-year-olds have the same chest circumference as sweet little infant babies.  Mine did, maybe yours do, too.  That’s good here because then the key numbers in the pattern don’t change.
• Like EZ says, slip the first stitch of every row.  I do it knit-wise, though I have never been quite sure this was correct.
• I eliminated the sleeve increases, and instead began with 18 additional stitches when I cast on.  This is explained briefly in The Opinionated Knitter on page 106.
• Overall there are very few changes to the original pattern, but the little tweaks I’ll discuss below make all the difference in the toddlerization process.  Keep your headlights on.
• Gauge: 19 sts = 4″ over garter stitch.  With the sport weight yarn I used [Hello Yarn Fat Sock, Marzipan, 2 skeins] this gauge produces a stretcy, loose and nearly lightweight material.

Ready?

Change #1 Provisionally cast on the specified number of stitches using a smooth yarn.  I did this by casting on (long tail, as usual) with cotton yarn, knitting a row, then joining in the sweater yarn and starting immediately with row 1 of the pattern.  Cotton is important for the cast-on if you’re using wool for the sweater, so that the provisional stitches are easier to free later on.

Solid green yarn above is the provisional cast-on.

Now knit the pattern as normal, through decreases and increases until you get to the point in the pattern that says “…work on center 90 sts. only, for 10 ridges…”

Change #2 This section of the pattern determines body length and for a toddler you’ll want it a bit longer.  I knit the center section for 14 ridges.

After picking up stitches along the edges of the section just completed continue again as written except for:

Change #3 Make the sweater slightly wider by knitting more ridges before the buttonholes.  I did nine ridges before the buttonholes and two after.  NB: I only made buttonholes on one side, but that is neither here nor there, and makes very little difference in the end.

Now for the fun part.  Or the tedious part, if that’s how you wish to look at it.  I thought of it as fun because without all the tedium you just have a pile of knitted garter stitch that doesn’t look like anything.  Although come to think of it that doesn’t sound too bad.  Moving on.

The order of things to follow, in brief.

• Cast off live stitches.
• Put all provisionally cast on stitches on threads until you are ready to knit them.
• Lengthen sleeves.
• Join tops of sleeves to back and seam.  Simultaneously.
• Bind off remaining stitches.

Here’s how I did it.

From the right side, and using a needle one size smaller than you used for the body, cast off using the i-cord cast-off method using three stitches (I used the one described on pg. 55 of The Opinionated Knitter).  Begin at the neck edge, continue down the front edge, around the back bottom edge, up the opposite edge, around the neck to where the sleeve joins the back.  Here, I’ll show you.

The star shows where to begin and the arrow shows the direction of the knitting (not that I needed to explain that to you, smart knitter that you are).  When you get to the end, leave the last three i-cord stitches on a holder.  Like this (except put them on an actual holder, not a needle, because the needle will indubitably fall out).

Now the time has come to liberate those cast-on stitches from so long ago.  Put the stitches from both ends of the cast-on row — the stitches before the first line of decreases and after the second line of decreases — onto separate holders.  That is where you’ll lengthen the sleeves.  The center stitches go on their own holder.

To lengthen the sleeve, join in the yarn on one end section and knit back and forth until the sleeve is the desired length.  I added thirteen ridges and probably could have done a few more.  Do not bind off.  Repeat on the other sleeve.

Oh, please don’t look so confused.  We’re getting there now.

You are ready to join the top of the sleeve to the back using a three-needle i-cord cast-off.  Count the number of stitches on the sleeve top (excluding cuff), then put a matching number of back stitches on a needle ready to join.  Begin at the star…

…and perform the magical trick known as the three-needle i-cord cast-off down the top edge until you reach the cuff.  Go back to regular i-cording around the cuff edge until all the stitches are cast-off.  Finish off the i-cord then sew it neatly where it meets the corner.  So far so good.

How beautiful is that seam?  And no sewing needle required!

But back to business.  Do the same for the second sleeve, but this time start at the cuff, i-cord cast-off around, then join the top sleeve stitches to an equal number of back stitches with the the three-needle i-cord cast-off.    You will also i-cord around the second half of the neck to join up with the beginning of these i-cord escapades. Thusly:

We’re nearly there now.  All that’s left to i-cord is the remaining live stitches at the back of the neck.

Ta-da!

Some Other Notes

• When you i-cord cast-off around an outside corner, add a row of i-cord that doesn’t connect.  This helps keeps those corners neat and square.
• If you have remembered to slip the first stitch of each row, it will be so much easier to do all the i-cording.  If you have forgotten, well there’s not much to do.  Use a tiny needle to pick up the stitches, maybe?  That might work.  Try not to forget the slipping.

I had such fun making this sweater and wrestling with the finishing details.  Please enjoy this little unvention of mine and let me know if you try it, have any trouble with the instructions or have any suggestions.  I bet if you toddlerize your own BSJ, you’ll probably do some unventing of your own along the way.  Yippee!

One last picture for good measure.
OH!  I almost forgot.  There’s one little “mistake” that I know you won’t believe me when I say I did it on purpose.  I did.  If you find the mistake you may award yourself 15 Cool Points.  Good luck.