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Ravichandra sindheRavichandra sindhe 

how to access child records in parent object using formula field

Khan AnasKhan Anas (Salesforce Developers) 
Hi Ravichandra,

Greetings to you!

Unfortunately, this is not something you can do with a formula. Formulas cannot read child records. The best result would be achieved using an Apex trigger or process builder. Or you can use this free tool from AppExchange:
https://appexchange.salesforce.com/listingDetail?listingId=a0N3000000B45gWEAR

If you have two objects, one called Parent__c and one called Child__c, that are related via a lookup/master-detail relationship then you will only be able to add a cross object formula to your Child__c object. Referencing a parent object from a child object is known as "Going up one level" which is totally handy in a lot of cases. 
But what you cannot do is have the formula field on the parent while attempting to pull in values from the child record. The reason for this is because of the nature of the relationship. When you are attempting to go down one level, you have to take into account that the underlying architecture allows for a parent to have multiple children.  This is known as a “One to many” (or 1:N) relationship in the database design community. 

A Master-Detail relationship has a couple of nice little extras that allow you to get record counts and other mathematical descriptions of child records based on limited criteria but overall just keep in mind that formula fields can go up and not down.

Reference: https://success.salesforce.com/answers?id=90630000000DAuMAAW

Refer this also: 
https://blog.jeffdouglas.com/2009/07/30/roll-up-summary-fields-with-lookup-relationships-part-1/

https://developer.salesforce.com/forums/?id=906F0000000BHvJIAW

I hope it helps you.

Kindly let me know if it helps you and close your query by marking it as solved so that it can help others in the future. It will help to keep this community clean.

Thanks and Regards,
Khan Anas