Hiya, Amanda! Welcome to planDisney!
Reading your question, I'm imagining you're asking about the instant ice packs that you might find in a First Aid Kit. If this is what you mean, then I think you are probably okay. I've brought one into the parks in my First Aid Kit before and had no problem. In general, the bigger concern is loose ice (or even ice in bags) that might become a spill hazard - hence why they are on the
prohibited items list. If you are meaning another kind of instant ice pack, that decision is at the discretion of the Disneyland Cast Members at the security checkpoint. They are very good at thorough checks and will let you know if they have concerns about anything you're bringing in.
Amanda, if you need to keep anything cool for a longer period of time, it might be wise to
rent a locker as they are in a covered area, less susceptible to heat. Lockers are also super convenient as they are available for the duration of the day, and offer in-and-out privileges. I was team no locker for a really long time until I realized just how handy they are for an all-day visit! Lockers are available for rent outside both parks, on Main Street, U.S.A. in Disneyland, and Buena Vista Street in California Adventure.
Thanks again for stopping by, Amanda. Please visit us again if you have any other questions!
See ya real soon,
Melissa