What is classed as ‘light bedding’? We are having a debate over a lightweight quilt that can be tucked in over an 18 month old in a cot.

Light bedding generally refers to light weight - ie bedding that is not heavy or multiple layered(or loose) that might make it difficult for baby to move (especially when learning to roll) or lead to
overheating.
Does your 18 mth old stay under the quilt or kick it off?
Many parents use a sleeping bag (with arms out) with no blankets or quilts at this age.
If you are concerned about your baby being cold you can add extra layers of clothing.
https://rednose.org.au/article/bedding-amount-recommended-for-safe-sleep