May in CA is unpredictable. As PP mentioned, you could get "May Gray." You also could get temps in the 90s. This January has been very warm (TOO warm IMO).
But the likelihood of humidity is very, very slight. So unless it is one of those relatively rare days of extreme heat, your DD will be okay in leggings no matter what. I often dress DC in lightweight long sleeves for DL, though they wear Ts in the summer months. The advantage of long sleeves is that you can pass on the sunscreen except for the face.
Be aware that Anaheim CAN get quite cool in the evenings, even in May. You may need a bit more than a light jacket at night.
If your DD is used to wearing sandals all the time, that probably would be fine comfort-wise. But her feet might get cold if the temp drops at night. I would recommend tennis shoes with socks. They will be fine during the day and will provide a bit more substance in the evening, as well.
I don't know what you're planning to take as far as bags and such. It is a pain to get to/from most parking at DL, but they do have lockers you can rent if you don't want to carry around jackets and/or other layers all day.
DS '04 "Boogaboo"
DD '08 "Lilybear"