My daughter is now 4, had just turned 3 when the pandemic started in 2020. We rarely took her out anywhere in the beginning to keep her safe at home. Sometime around July / August 2020 we started going for walks with her wearing a mask to introduce it to her because she was going to go to PreK part time in September. I got her many masks all different with her favorite characters on them. I also wore some with characters. To her, it became like a "dress up" activity. That is how she got used to wearing one. Then, she wanted to coordinate them with her clothes. lol Wearing a Minnie Mouse mask with her Minnie Mouse shirt, or Elsa mask with her Elsa shirt, etc. Now she thinks it is part of playing dress up. But, she keeps them on all day when she is in school. She only goes 3 days a week though.
When we go to the store, she wears her mask without a problem. I explained to her that the reason that we have to wear them is that some people are sick, sometimes they don't know it yet. We wear our masks so that we won't catch the germs, and so we won't get sick too. Obviously, she doesn't understand the depth of how bad things are or were, but she sees that everyone around her at school, and when we go to the store, everyone is all wearing a mask. Then, when some comment on how cute her character mask is, she wants to show it off more. Like the way she was pre-pandemic where she would have pride when someone says something nice to her about how she looked when she wore her princess dress in public.
Every kid is different, and will be affected by it differently depending on their circumstances. So, only you will know how best to help your child. I think most kids are going to be resilient.
We are going to bring her in August 2021 to Disney, I think that we will use our discretion in doing park time in moderation, going back to the resort to enjoy relaxing time there. Not sure how things will be in August though.