I gave my son my old watch for Christmas (a nice classic Swiss Army one) but I cant find a strap with a 20mm width that will fit his wrists. I had bought a short velcro one but it’s still too long, and all the kids and ladies straps I can find are too narrow.

Anyone know where I can find a 20mm wide band that will fit a 14cm wrist?

Get a NATO nylon one, they can be modded just by using a hot wire or something to melt extra holes. If you get the ones with double keeper rings, the extra material can be doubled back on itself.
http://www.amazon.co.uk/NYLON-WATCH-Choice-Styles-HARLEY/dp/B00DD7X70C/ref=sr_1_1?ie=UTF8&qid=1420052433&sr=8-1&keywords=nato+20mm+strap
This pic shows the strap folded back and pushed through the double keepers:

Ive had them on several watches, including my G-Shock using adaptors, and they’re great, very comfy, and easy to keep clean.

+1 for a Nato, also Rhino and Zulu similar, wide range of colours and stripes too.
You could also try an independent jewellers as they may have a different range of straps form the high street chain shops?

Maratac straps are generally thicker material, which may make a difference. Personally I prefer the Maratac straps, but they’re not so easily obtained, IIRC I got mine from here:
http://www.broadarrow.net/maratac.htm
Huge range of sizes, colours, styles.

Thanks for that. Will check them both out.

try a tropical perlon nato / zulu strap. same idea as above but woven and softer better for a youngun id say also has the plus of not needing to make new holes as he grows as the buckle tab just fits through the weave.

Here on ebay from a uk seller, google may throw up others
http://www.ebay.co.uk/itm/Perlon-Tropic-Nato-Style-Plaited-Watch-Wristwatch-Black-Strap-Military-/281148878616