What evidence supports it is unsafe for a soft toy to be attached to a dummy for an 18mth old to sleep with (ie a bunnys head with attached blanket approx 40x 40cm in size attached to a dummy)

Red Nose information about soft toys for use as a comfort & to aid settling includes the following.

“Keep soft toys out of the sleeping environment for babies under seven months of age because they may cover the nose and mouth and interfere with breathing.

The risk posed by suffocation by the presence of soft objects in the baby’s sleeping environment outweighs any benefit to the baby from a soft toy. It is therefore advised not place soft toys and other soft objects in the cot for babies under seven months of age.

Seven month old babies are more likely to explore objects in their sleeping environments than younger babies. Some babies over seven months of age may appreciate a small object such as a soft toy to provide comfort and connection (transitional object) during times of separation from their parent.

Keep the cot free of large soft toys, pillows, bumpers, activity centres and anything else that could be ‘stacked’ to assist a young child/toddler to climb out of the cot.”

For full information & references - https://rednose.org.au/article/soft-toys-in-the-cot