How about a Citadines Aparthotel? I have a toddler and while I have not taken him to Paris (yet) I find when we travel with him it's sooooo much easier to have a kitchenette. The tantrums averted with easy access to apple juice and sippy cup washing facilities are worth their weight in gold.

Near Gare du Nord I would probably go with Citadines Opera - there are places to buy small groceries nearby and wide boulevards for the little one to toddle down!