The good news......there is no bad choice.....all three days have their positives and negatives for opening each park. So what ever choice you make, prepare yourself for magic.
As a visitor for 50 years I cringe at suggesting DCA first and have never visited DCA first. But I am Monday-Thursday visitor and that is a better fit for my pilgrimages.
And once you purchase a Disneyland guide book at the book store or on line and read it three times you will have a grasp of the variety of suggestions offered here on this site.
So here my reasons for DCA first.
1. You want your child to be confident. Dark rides can be a problem for some children, not all children, but some children. You will never know until after that first dark ride if your child is in fear or having a blast. Regardless of the child you know right now, you will not know that reaction until the first dark ride is over. Mermaid in DCA offers a bright, happy story until a dramatic, semi scary ending. All dark rides in Disneyland Fantasyland start dark and end dark and scary. Mermaid Movie can be shown at home and prime the child for the joy of seeing Ariel. If the child responds with joy to the dark ride, then super, you will have a great two days at Disneyland. If the child responds to the attraction in fear, then off to Bugs Land you travel, to a very friendly child environment. soothing, simple rides outside much like a state fair. Then travel to Animation building to take in more child friendly events to build courage. Then to carsland for more outdoor fun, though limited to just one ride.
2. Carsland is where is the guide reading comes in handy. The two adults can do child swap or single rider for RSR while escorting the child around for sight seeing the in the wonderful area. Many wonderful photo opps in this area. Though I would discourage Racers for first time visitors as it has a habit of not functioning on a regular basis and could consume valuable time spent elsewhere. How did you feel the last 24 hours waiting for this site to come back up? You can very well have that feeling about waiting for RSR racers to boot up to admit visitors at any given hour.
3. Stroll around , take in other suggestions in the guide, more child swapping, outdoor places to eat. many wonderful photo opps again while the other adult explores hard core attractions.
4. it is a nice slow pace, wide walkways, open visuals allows you to measure the strengths of the child.
5.With the park hopper, you can always venture into Disneyland later in the day to soak in the sights to prepare yourself for the next two days. Main street has wonderful sights and great to asorb them so that the street is not a distraction first thing in the Wednesday and Thursday mornings.
6. The reason that I am suggesting using early entry for Thursday, is that you will have time to research your gude for specifics while resting Tuesday and Wednesday night in your lodging. Research will help with fastpass collection, child swap, meet and greets, and areas missed because just didn't see the areas i.e. Toontown and Thunder Ranch and the Pirate Island, all very child friendly. Getting your feet wet Tuesday in DCA, will help with the Disneyland plunge on Wednesday and Thursday.