Jet Blue considers them assistive devices and doesn't count. We had me, wife, and DS, DD (not going to WDW though)...
We checked a stroller under DS and a carseat under DD. We had an extra space for luggage returning as me and the wife shared 1.5 bags and the DS took up the other .5, while the DD took a bag.. We did use that last space on return..
great thing about JB is they don't charge for the 1st bag per ticket.. also, I had my laptop bag, which was a personal item and still was able to bring a carryon.