We took my 3yr old grandson for a passholder preview day, and then again yesterday afternoon for a little bit.
We don't wear masks on a normal basis so he isn't used to them. The passholder day, he did amazing! No fussing no complaints, kept it on as instructed all day, didn't take it off until we told him he could in the parking lot. Yesterday he was napping when we went in, they did not temp him, nor ask about his mask. Nothing at security about it. While he was sleeping, we just let him be. We made the mistake of waking him up and he had a massive meltdown and wouldn't wear the mask. It was horrible and embarrassing, the park was so slow and quiet that it made the tantrum even louder. No one approached us about the mask though. He finally calmed down and wanted to go on the seuss coaster but wouldn't wear the mask, they did not ask him to put it on until we were sitting on the ride. And even then they just let it go when he pulled it down after we put it on. We kept pulling it back up throughout the ride and eventually he just kept it up. He wasnt overly pleased about it but we told him no mask, no rides. So he eventually just stopped fussing about it and wore it.
We are local so we were only going in for an hour or so before the rain started anyway.
They also make you put on hand sanitizer before the rides. Like the mask, it isn't optional so that is something to consider if little ones don't like it.