Looking for a 'secret' texting app?

My girlfriend of three years and I are trying to look for an app that we can text over, but it's not at all an obvious texting app.

Her parents recently found out that she was disobeying them and seeing my family, who they don't like and now we're not allowed to contact eachother at all.

We're using Google Docs and it's awesome to talk to her again, but it could be alot more convenient. Her electronics are heavily monitored and that's why it can't be obvious, like maybe even a secondary function of an app.

I've got the Android Note 3, she has the Iphone 4s

Google docs! I love it.

Just use a simple app such as Line, sign into a private account when you want to communicate. That way, if anyone checks you can be signed into a different account.

Keep using Google Docs and making it editable by her so you can communicate. That's such a brilliant idea. If it's such a touchy situation, I wouldn't risk anything between you two for the sake of convenience.

Let her get a secret phone to text on