The easiest and the funniest way to go to Ancol is by taking a bus way (though you can only access this during the day). Take any bus way connection to Kampung Melayu, and from that there is one direct connection to go to Ancol. Moreover, it is cheapest, you pay only 3,500 IDR, and it is faster.
Once I took my twin 'devil' nieces for swimming to Ancol Beach. We went by bus way, and for kids under 5 years old, you pay half-price only. The twin seemed to enjoy the trip very much. Their eyes lingering a long the road, looking for the scenery outside the windows. I love the big and wide windows, cause it gives me a senses of being in an open area (though I am inside the bus). I don't like being inside a very small room. It takes almost half an hour as the bus had to stop in some places. But overall i t was a good trip. The bus will stop inside Ancol area, and from that there is a free-connection bus that will take you to your destination; Ancol beach, Dufan, etc. Try to go to Ancol by busway!