Sleeping Bags and banging arms on crib sides - what's the answer?

[forevared]

I'm about to transfer ds2 into a sleeping bag as he's starting to wriggle out of his swaddling blanket during the night but on the odd occasion I've used for a daytime sleep he just bangs his arms on the crib sides and wakes himself up.
I wondered if cot bumpers actually work to prevent them from waking, anybody know from experience or does anyone have other tips?

DD sometime startles and flails so we put a bumper in to try and at least pad the clonk, but tbh a teddy bear either side of her works better.

Ikea do the best bumpers (here) as they go right round the cot rather than just round the head end.