For a Nursery with 0-5 in attendance is there a maximum height for a crash bar attached to the fire door?

Panic bars are fitted at a standard height – between 900mm and 1100mm in accordance with BS EN 1125:1997. This height should be taken into consideration when specifying mid rail heights. Where it is known that the majority of the occupants of the premises will be young children, you may wish to reduce the height of the bar and the midrail.

However in your situation I would imagine there will always be an adult with the children who will most likely required to assist the children to escape and so they are the most likely to have to open the door not the children.