My dd is six months and is a real wriggler. I put her in her cot and she shimmies over to the side because she prefers to sleep with her face shmushed against the bumper. She then rolls onto her side, facing that bumper and gets her top leg and arm caught between the cot bars above the bumper. This pisses her off buy as soon as I untangle her, she does in again. I can't put her at the other side because she immediately rolls onto her tummy. I don't want to use a restraint or positioner put swaddling. So I'm currently squeezing her into a Moses basket because she sleeps better. Can you get bumpers which go all the way to the top of the cot so she can't get her limbs out?
Its a bit inconclusive, so say never use and others say use until baby can sit up. But sleeping with her face smushed against the bumper whilst her limbs are pulling down on it sounds like one hell of a smothering risk.
Really, she's going to struggle to hurt her head whilst rolling around the cot that comes later when they pull themselves to standing in it but she could well get entangled and smother herself.
You can buy cot netting that you attach to the outside of the cot, that will stop her arms and legs going through the bars but is much safer than cot bumpers. I'm on my phone just now so can't link but I think you get it from Amazon or Mothercare.
Cot bumpers are a smother and entanglement risk, manufacturers advise to take them off when a baby can move about in the cot. The Foundation for the Stusy of Infant Deaths, the NHS and the Child Accident Protection Trust all advise against using them. Check out the safe options you have been suggested instead.