Me and my family (two adults, one 14year old) are going to visit London for 5 days (Saturday to Wednesday). Our hotel is near Crystal Palace in zone 3, but it has only Overground connections. On Saturday, I will travel from Victoria to City airport, pick the rest of my family up, we are then going to the hotel to leave our luggage there. We plan to take a train to the city every morning (probably before 9:30), and return in the evening. On Wednesday evening, we need to go to Gatwick airport.
Since the fare system is in my opinion ridiculously complex, I need your help to find the cheapest fare? Which option is the cheapest for our plans? Oyster cards? Travelcards? 7-day travelcard? Which of them have child fares? We don't have contactless credit cards.