On both of your days, DCA has early entry and DL does not. DL has early entry on tuesday, thursday, and saturday, and DCA has early entry on monday, wednesday, friday, and sunday. So since you are going on a sunday and monday, DL won't have early entry at all. Are you staying at one of the three onsite hotels? If so, you get access to early entry at a select park everyday (see schedule above), so you can enter DCA early monday and tuesday.
On the other hand, you don't get any type of early entry at all if staying offsite. You only get one if you buy a three day or more day ticket, and they are only valid at DL.
I think your strategy of hitting DL on a monday works well. Just make sure you arrive 50 minutes early ( or even earlier if you really want to be at the front, as it is super busy at DL when you are going, albeit not as busy as the period between Christmas and New years eve). Monday's are less busy than weekends, and DL has more stuff to do than california advenure, so you will need that slight lack of crowds (but it will still be crowded!). As for DCA, visit it on a Sunday and take advantage of extra magic hours if you stay onsite. There really is not any difference in crowds in DCA if you are participating in extra magic hours. If you stay offsite, still go, but avoid carsland in the early morning, as it will be already be packed with onsite early entry guests before you cross the gates! Fortunately, the other areas, including those open to EMH guests, are not affected. If you have park-hoppers, you have much more flexibility for such a short trip. If one park feels crowded, just hop to the other one! So easy!
I hope that you have and amazing Christmas trip!