Hey there Rosemary,
Welcome back to planDisney and thanks for another great question!
Like you, I also have a 4-year-old who absolutely loves Anna and Elsa. It is always a must to stop by and say "hi" to them on any trip we take to the Happiest Place on Earth.
Actually, my recommendation for your best bet to meeting both of them is in Disney California Adventure Park where you can visit
Anna & Elsa's Royal Welcome in
Hollywood Land. Depending on the time of day that you visit, there may be a bit of a line inside the animation building, but you'll get to enjoy the beautiful art of Disney movie displays that are on the screens surrounding you while you wait.
We love this meet and greet opportunity because you get some extra one-on-one time with Anna and Elsa, which our kids always enjoy.
Depending on your ticket type, you may want to consider adding the
Park Hopper option, which would be my recommendation if you can swing it, or just swapping your
theme park reservation to Disney California Adventure Park for the day if this is your number one objective for that day in your trip.
Otherwise, a great way to get an idea of which character experiences are available in the parks on any given day is through the
Disneyland Resort mobile app. If you tap the map pin icon then select "Characters" you'll see all the planned character experiences for the day in each park. I actually like to tap the "Show List" view to find specific characters and then you'll be able to see general details about where you might be able to find them.
Thanks again for your question, Rosemary! I hope you have a wonderful time on your upcoming trip.
Have a Magical Day,
Greg