I prefer flat-front pants (where there is only elastic in the back) as I think they look a bit snazzier, but you could erasily do this with a full elastic waist. Here's my quick and easy method to convert thrifted mens pants into toddler/boy pants:
Men's jeans or pants
Elastic to fit in waist-band (I about 9 inches of 1/2 inch wide elastic)
Step 1. Create pattern from an existing pair of toddler pants (preferably flat-front pants) - for a great tutorial on this, check out the one here at MADE (a fabulous blog, if you haven't check it out before!).
(Note: the pattern pieces won't line up perfectly with the curve of the centre seam, but it shouldn't make much difference to the fit of the pants.)
Step 3. Flip the pattern piece to the other side of the centre seam, and repeat, so you end up with a single piece for the front of the pants:
Step 4. Repeat for the back pattern pieces on the back of the pants:
Step 5. Remove any studs, labels or excess pocket material in the seam allowances that will be difficult to sew through. I removed the leather back label, the small coin pocket that had one side seam showing, including the stud. I tried to prise off the stud with pliers but ended up just cutting it off as it was in the seam allowance).
Step 6. Pin together the front and back pieces, with right sides of the material together, in the inner leg/crotch area, and sew together. I used my serger/overlocker first (yes, I only have white thread for my overlocker!), and then sewed the seam with brown thread on my sewing machine to make sure the white thread doesn't peep through. (Note: I prefer to overlock before sewing as I'm not very confident with my new overlocker - do it the other way around if you prefer.)
If you don't have an overlocker, do a zig-zag stitch in the seam allowance before sewing with a machine, or just sew the seam and don't worry about finishing off.
Step 7. Cut a length of elastic approximately an inch longer than the width of the front panel, and put a safety pin in one end:
Step 8. Thread the elastic through the waistband of the back section of the pants, pinning the non-safety-pinned end, so you don't pull it all the way through:
Pin the elastic at both ends with a small section hanging out of the waist band (to make sure you sew through it).
Step 9. Pin the front and back pieces of the pants together for the side seams, and sew, as per the inner leg seam (ie I serged/overlocked, and then machine sewed again). I sewed through all pocket material to create little pockets in my pants.
Step 10. Double-fold over the hem for each leg, pin and sew with your sewing machine (or hand-sew a blind hem, if you prefer).
Step 11. Turn your pants right-side out and admire your handiwork: