I thought this pattern was hilarious and perfect for a 2.5-year old boy.

I loosely followed eartheo’s mods to fit a toddler.

Used size 10 needles for the body, 7s for the fins.

For the tail, I initially knit it as per the instructions, but I didn’t like the way it looked (wrong sides showing), so I ripped it back, and fudged my own pattern, knitting in the round, to get it to look like it does in the pattern pictures. MUCH better and neater looking.

In case anyone is interested, here’s what I did:
Row 1: (k2, M1) to end (32 st)
Row 2: k all st
Row 3: (k2, M1) to end (48 st)
Row 4: k all st
Row 5: (k4, M1) to end (60 st)
Row 6: k all st
Split stitches in half, with split along line of middle of mouth (hope that makes sense). Put half of st on a stitch holder.
Working with first half of stitches, start round at split. I did this using magic loop, but it would work fine on DPNs, obviously.
Row 7: k1, ssk, k until 1 st from halfway point (i.e. k11 for this row), M1, k1, place marker, k1, M1, k until 3 st from end (i.e. k11 for this row), k2tog, k1 (30 st)
Row 8: k all st
Row 9: k1, ssk, k to marker, slip marker (sm), k to 3 st from end, k2tog, k1 (28 st)
Row 10: k all st
Row 11: As row 7 (28 st)
Rows 12-16: As row 9 (18 st)
Rows 17-19: k1, ssk, k to 3 st before marker, k2tog, k1, sm, k1, ssk, k to 3 st before end, k2tog, k1 (6 st)
Row 20: As row 9 (4 st)
Break yarn. With darning needle thread yarn through remaining stitches and pull end through. Secure end.
Repeat rows 7-20 with other half of stitches.

I hope that made sense! I’ve never written a pattern down before, and I kind of did this on the fly writing things down as I went. I’m sure there’s an easier way to do the tail, but this is how I did it! :)