25 drops of fenistil drops for 6 months baby. ...is it safe?

Yes, use it properly. the drug is safe for a six month old, the range of daily dose for infants is 0-30 drops per day, check the package insert and follow directions. Read also to familiarize yourself with side-effects - GI complaints, dehydration, etc.
Too much. fenistil is not available in the US, but from what I've been able to find on-line that is way too much! Read the package directions or discuss with your doctor before giving.