Surely this is a fault on your particular pushchair? I had this for my twins & they were in it till 3.5 with no problems. Unless you have very large dcs I would contact out n about & ask for replacement straps.
We have the V1 Nipper 360 Double and the safety harness has been getting steadily tighter. My girls are only 16 months old and already I can't fasten the harness around their hips, so they aren't strapped in anymore which doesn't feel very safe. Has anyone else experienced this and found a solution? I have found harness extension straps for other brands of pushchair but they don't fit.