Can I disable everything except talk and text?

You really can't. You can regulate lots of things but if you want a phone that will only talk and text, you're better off getting your child a basic phone.

But, if you don't think that's an option, see here for what you can do:

Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

IdrisSeabright offers you excellent advice (as usual).

Here’s one more alternative to consider:

You may restrict the iPhone to talk or text (exactly one application) as follows:

Use Guided Access with iPhone, iPad, and iPod touch - Apple Support

sberman wrote:

IdrisSeabright offers you excellent advice (as usual).

And, as usual, you're very kind!

Here’s one more alternative to consider:

You may restrict the iPhone to talk or text (exactly one application) as follows:

Use Guided Access with iPhone, iPad, and iPod touch - Apple Support

Good suggestion. I forget about Guided Access.