We did the two day tour and I can't recommend TJ Tours highly enough! The whole experience was amazing. We were in a mini bus for 10 people (with wifi and water) and we went absolutely everywhere! Our guide Alex was lovely and so knowledgable - she got us in to all the busy places without queuing, she even knew where the less crowded loos where! Her art knowledge in the Hermitage was outstanding.We even went down a subway (a fabulous experience!) as the mini bus was able to get up all the little side streets and dodge the traffic! The first day went from 8.30am - 6.00pm and the second day from 7.30am - 4-00pm. We were on boat trips and driving in the mini bus so it didn't feel too long and we were back at the boat in plenty of time.
We were charged 240$ each (with a discount for being on Cruise Critic) and we asked for a price in Rubles before we went and took the cash we had exchanged using John Lewis including cash for tips. In fact we got a better exchange rate on the day and so spent what we had left in the souvenir shop at the end of the trip where you pay (they seemed to give the best value for money too!). All in all a fabulous trip. I would expect that if you wanted a shorter day you would have to work out what you didn't mind missing and that depends on how young your children are. The Hermitage was amazing but very busy and we did see some very bored looking youngsters trawling around there. They would probably love the boat trip and the hydrofoil to Peterhof and the subway ( as long as they are ok on the longest escalator in the world!).
Hope that is useful for you!