This worked brilliantly for us:

http://www.amazon.co.uk/Houdini-Stop-So ... B0044VUXGW

We had the same problem and overall I wanted him to have shoulder straps so used a small dog collar with a plastic release clip that you squeeze. My child couldn't undo it and it pulled the straps closer so he couldn't get his arms out… I am sure there are probably safer ways but it worked well enough that he soon got bored of trying...

Does anyone us a chest strap to prevent Little One (2.4 years) from getting his arms out of the proper restrained position? I am really uncomfortable with him not having the benefit of shoulder straps in the event of a sudden stop. It takes hardly any time before he's out of them.
It's a Maxicosi Pearl if anyone wants to compare notes.
I've read that they're not legal/advised in the EU, as it is an additional clip to have to deal with in an accident. Other sites (US often) claim that they are unsafe due to unbalanced strain...
Any thoughts or solutions?