My girl kept getting her arms out we have a plastic buckle that pulls the straps together across her chest to stop her getting her arms out
Have you had her fit checked recently? Being able to get arms out etc is often because the fit isn’t right.
We have the be safe collector too, it’s crash tested and recommend on the car seat safety pages, our little one doesn’t even attempt to get his arms out now totally admitted defeat 🤣
Make sure the headrest is in the right place. The straps should come out level with her shoulders. If that isn't the problem, the be safe belt collector is the only crash tested/approved device of its kind on the market
We have the be safe belt collector which has worked for our little guy