I have to warn you, these are so cute you may not know what to do.
I purchased the Period Impressions pattern for Boy's Button Fly Pants (Trousers) in two size ranges. Those I made today are size 4, and fit my 3.5 year old, big boy quite well. I used cotton wide wale corduroy and rivet buttons.
The pattern could use some editing help, and was really hard to figure out at first (or second, or third) glance. It was also my first time constructing a fly of any kind. I am so pleased with these, though. Cuteness incarnate.
Toly loves them. They are green (a huge plus for Toly) and comfortable. Things I would do differently? First, I would use a real seam finish. This pattern calls for no fine finish on the inside at all! They will be much more durable with french or flat-felled seams. I just followed the rules this time, but next time they will be better. Also, I think this was a difficult fabric for my first pair.
Tristan's first pair is next. He measures a size 8.