Zarava - This is the third AHR Meridian we've had. First two were replaced, for the exact same fault, and then we've had the third one do the same thing. Plus DS has been able to get his arms out of the restraints since he was about 13 months old. Now he does it so often I am constantly watching him through the rear view mirror. When I contacted Safe n Sound about him getting his arms out they basically told me it was impossible, but I have a friend who has one as well and she has the same issues with it as I do.
I bought a Securap from the RACQ as DS2 can easily get out of both his carseats - we have a safety 1st in one car and a Meridian booster seat in the other. The securap is quick and easy for me to get off so it wouldn't be an issue should i have to get him out of the car quickly however, he can't undo it himself. I know that you can also buy the buckle guards from RACQ as well.

He was getting out of the straps by pushing his body to one side and releasing an arm and then the other one comes out easily.