Feet dangling can cause distraction and make children feel unsafe. Put a box/step under feet to help make them feel safe and sit up.
Look how the child sits on the chair. If they are sitting like the child in the picture below then they may benefit from having support at their sides? If so use a chair with arms or place them next to a wall so that they have support on one side.
It’s helpful to keep a child in the highchair for as long as the child is able to safely fit. Highchairs support when eating. Children often feel safer when their feet are supported.
The picture above shows a good way of introducing children to eating as family if they are unable to tolerate a highchair to start with, try having a picnic on the floor with the family and then try transitioning to a chair or highchair.