We're currently working on the iOS port. We plan for this to be ready for the upcoming 2018-2019 school year. Once this is up and running, you will have to make no changes to your app--just download the iOS Companion app from the store!
If you want a quicker option, Thunkable X is a solution that allows you to build cross-platform apps, but I would be cautious. Make sure you read the fine print.
Let me know if you have more questions!