Trojan Sparta Espag Handles Locks Breaking Problem?

I have replaced all of the UPVC window handles in my home with Trojan Sparta handles. They have only been fitted about 5 months, but several locks are broken. The key turns, but the handle button will depress whether "locked" or not. (No matter what you do you can always press the button.)

Has anyone else had this problem? I have young kids in the house, but I wouldn't have thought they could put so much force on the button that they could smash the inner workings. I can't work out why they are breaking.

Can anyone recommend any others which are known to be more hard wearing?