A 'Trial lesson' system

There are steps one can take to reduce the likelihood of a child joining a pre school dance class who is 'not ready'.
'Not ready' in this context means that the child is unable to concentrate, is reluctant to join in, easily distracted, not listening to the teacher, running around the room etc .

At the first point of contact when a parent enquires about their child joining a dance class the teacher should ask the parent if the child is already attending a nursery, playgroup etc. This will indicate whether the child is familiar with the concept of participating in an activity with other children (without the parent) in a more structured setting.
Operating a 'trial' lesson system is extremely helpful.
This system would be explained to the parent at registration. A trial lesson date and time would be confirmed. During the trial lesson the teacher would be able to assess the child's receptivity to the class, general level of enjoyment and participation. Based on this knowledge the teacher would then be able to inform the parent whether or not the child was ready to join the class on a regular basis.