Can we delete the Lookup list item if reference is present into the child list.

Yes. If we have not selected Enforce relationship behaviour check box will created lookup column in child list. If we ticked the Enforce relationship behaviour checkbox Restricted deleted - In this case it will prevent the deletion of an item in the target list, if it has one or more related item in this child list. Cascade deleted -In this case if item in the target list is deleted, it will deleted all the related items in child list.