When you enter the concourse at Gare du Nord, turn left and look for the 'RER' signs. (RER trains provide a 'limited stop' service, so they're quicker than the Metro). Look for Line B, with the signs indicating travel towards Robinson and Saint R�my l�s Chevreuse. Take the train for 2 stops to St-Michel Notre-Dame. Explore the area around there, (including viewing Notre Dame from
outside) and possibly wander into the adjacent Latin Quarter for a bite to eat. (The area is packed with caf�s, bistros, restaurants and fast-food outlets). Then go to the quay which is just across the Seine tributary from Notre Dame, to board the Batobus:
http://www.batobus.com/english/8escale.htm
That's a hop-on, hop-off river bus service (which is far cheaper than the many guided tours on offer and much better with an excited youngster, who'll want to talk about all of the sights, rather than listening to a boring guide).
You could break your journey but I'd suggest staying on all the way to the Eiffel Tower. (The journey takes about an hour and there are loads of sights to see. Make sure that your 6-year-old has the camera strap around his/her neck, so that it can't fall into the river, and then put him/her in charge of taking the photos!).