(Consider saying that some children need help to be able to sit still and concentrate, whereas others don’t. The children that already sit well at mat time can help by role modelling for the children who find it challenging.) Also consider that It could be a reward for other children who are showing the desired behavior.
CONSIDER: Some children may not be developmentally ready to engage in a group experience. Consider a calm area containing fidget toys where the child can sit and play instead. Always provide the child with a choice; but make your expectations clear. E.g. Would you like to sit in the Calm Area or on the mat?