Hiya, Jazmin!
Welcome to planDisney!
Mickey told me to tell you that he's proud of you for being a super duper planner for your upcoming Disney day. While
there is a list on the Disneyland.com website of the items that are not permitted inside the secured area of Disneyland Resort, it's not completely comprehensive. I personally think that you'll be fine with a tens unit (which I need to admit I just Googled), especially if yours is on the smaller size and is battery-operated. Medical devices of all shapes and kinds are welcome, and I can see this fitting into that category. I think you'd also get bonus points if you brought it in its original box just in case anyone had any questions.
Some items are up to the discretion of the security team inspecting your personal property. I once accidentally left a bike lock in a forgotten pocket of our jogging stroller that I used for everyday trips in our neighborhood; even though locks aren't listed as an item that isn't welcome, the Security Cast Member shared that their team has the ultimate say, and because he found it he had the authority to not let it in. The good news is that if this happens to you, you'll be given the option to return your item to your vehicle or
Disneyland Resort Hotel Room for safekeeping, or you can turn it over to the security team who will dispose of it for you. But again, I do think that this will be allowed in the parks as it is a medical device.
I hope that this helped to clEAR up your question, Jazmin! Be sure to follow the
official planDisney Panel Instagram account for even more tips and tricks. And if you have any more
Disneyland Resort planning questions, We're All Ears!
~Kristen N.