My dd is in the britax isofix duo at the moment (2yrs4mths) and is a complete houdini in it (or out of it I should say). She started taking her arms out of the shoulder straps some time ago and we solved that for a while by putting a bulldog clip accross them. She can now get that undone, and also she is now able to undo the buckle at the crotch, so I am stopping every few minutes to strap her back in.
I tried her in my ds's spare booster seat today, though she hasn't reached the minimum weight yet. She kept the seatbelt on but when she was asleep she was flopping all over the place!
I have been to halfords, mothercare and the local baby shop but they all very helpfully laughed and shrugged their shoulders
Does anyone have any experience with this? Did you find any solution?