There are so many good tips already. I can just tell you what we did last time we were in St Petersburg. We were there for 2 days. If I am not mistaken from 8 am to 6 pm the next day. When we arrived, they let first the passengers go with tours booked via Disney. It took about half an hour before we could go. Our wonderful tour guide was waiting for us and she told us she would be our guide for both days. She was great with kids, very considered. In those 2 days, we didn’t do to Peterhof. We were more kids concentrated. So one day we did a special kids tour in the hermitage and we took the kids to paint nesting dolls. In the evening we went to a Folklore show (I personally wanted to go to the ballet, but that would be very boring for the kids.) Next day we had a general go around the city and saw the Church of spilled blood and the grand maket Russia, which was fun for young and old….
We could have done more in those 2 days, but I didn’t want to overdo it. We will go back once they are a little older. There is a lot to see in St Petersburg. We couldn’t be happier with Anastasia tours. All went very smoothly.