I wouldn't necessarily designate 1 day specifically for DL and another for DCA, but I would agree that 3 total days is a solid start. Since park hopping is so easy at DLR, I would not spend one day at one park, but rather, hop back and forth to maximize my time.
No matter when you go in December, 3 days minimum I would say for a first time. 4 or 5 would be ideal if you want to take it easier or at a slower pace, so with younger kids this might be the better option.