I've done this on almost every pair of trousers my kids wore when they were little and this is the simplest way I found of doing it. You do need a sewing machine but mad sewing skills are not required.
The jeans that are too wide
Elastic (about 2 - 3cm wide is ideal, just make sure it's not wider than the waistband)
Cut a little slot on each side of the jeans, on the inside, just in from the side seams. Make sure you cut to one side of any belt loop and only cut through one layer of fabric.
Take a length of elastic just a little bit shorter than the jeans are wide.
Pop a safety pin on one end.
Then start poking the elastic through one of the holes you have cut.
Once you are about half way along, stop and secure the other end, just use another safety pin to stop it disappearing (yes, I've done that and you just have to pull it right out and start again!)
Keep going, threading the elastic through until it comes out the other end.
Now you want the elastic flat and on the inside, but you don't want to loose the end. Remove the safety pin and carefully push the end back inside the hole.
Use the safety pin again to secure it.
Then do the same at the other end.
Now you just need to fire up the sewing machine. I stitch either side of the hole back and forth a few times, then do a few lines of zig zag to close up the gap.
It looks like this on the outside.
That's it, finished!
It's actually only bringing them in by a couple of inches but makes a big difference to the fit and comfort. I don't think I could manage to take the waistband off, take in the side seams and reattach the waistband, and until the kids are adult sized when you can buy jeans by waist & inside leg measurements, this seems to work!
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