Infants sleeping in the same bed as:

OK, so there's a bit of terminology here. Cosleeping seems to mean when the infant sleeps in the same bed as the Mom. Bed sharing seems to apply if the infant sleeps in the same bed as any other family member - e.g. parents or siblings. Co-bedding seems to be when the infant sleeps in the same bed as another infant, usually a twin/triplet, etc... However, not everyone uses the terminology in exactly the same way, so it's probably useful to try all three.

bedshar*[tiab] OR  bed-shar*[tiab] OR cosleep*[tiab] OR co-sleep*[tiab] OR cobedding[tiab] OR co-bedding[tiab]


Which you can combine with MeSH such as:
"Infant"[Mesh] OR "Infant Behavior"[Mesh] OR "Infant Equipment"[Mesh] OR "Infant Care"[Mesh] OR "Infant Mortality"[Mesh] OR "Infant Death"[Mesh]

For the twin co-bedding question, these may also be useful:
"Multiple Birth Offspring"[Mesh] OR twin*[tiab] OR triplet*[tiab] OR quadruplet*[tiab] OR quintuplet*[tiab] OR "multiple birth"[tiab] OR "multiple births"[tiab]

CINAHL also has:
MH "Bed Sharing"