set<Id> accIds = new set<Id>();
for(Account acc : Trigger.new)
{
   accIds.add(acc.id);
}
//query the contact records now from currently processed Account Ids
List<Contact> lstContact = [select id from contact where Accountid in : accIds];

List<Contact> lstContact = [select id from contact where
Accountid in : Trigger.newmap.Keyset()];