why we cant increase the no of end device to big numbers like 50 or 100??
is there any reason w.r.to 802.15.4??
or anything else??
because i am using an application which requires 50 end devices and one coordinator only??
how can i make it??
is there any way to achieve it with out using routers in between?? 
What you are describing would be best done under an 802.15.41 network and NOT a zigbee network.
In a Zigbee network, an End device populates a table that is stored in the modules Flash. Since the module has Limited memory, there are only so many entries or End devices you can have.
thanks for your reply but i want to increase no of children by using ZigBee only is there anyway of doing if??
Without wiring your own firmware, re-certifying with the FCC and CE, it is not possible to increase the number of end device children supported on a XBee ZB coordinator.