Including unofficial teaching experiences in CV
In the unlikely event that there is a formal requirement on teaching experience, it’s up to the them to decide what fulfills it. In that case, it may be worth contacting them to see what kind of proof they require. But if that were the case, you wouldn’t even be considering omitting this experience, so I presume this does not apply.
For everything else, I would not worry too much about what is an “official” experience and what is not, but rather on whether it is something that demonstrates or enhanced qualities relevant to your success as a PhD student.
The only way something can be negative on your CV is if it is ridiculously minor (say, you only tutored for one hour), your CV is already filled with more powerful stuff, or it is obviously irrelevant to the position you are applying to.
You have to be the judge of the former points, but even if the program does not explicitly mention teaching experience as something it values, they should consider it relevant:
not only for its own sake, as you may be required to teach during your PhD, but also because it enhances your communication skills and requires a deeper and structured understanding of the subject matter, which in turn is particularly relevant if you taught advanced topics or they were related to the program’s subject.
If it was voluntary, it additionally evidences commitment.
If you list your teaching experience, make sure that whoever reviews your application can understand what you did there:
What did you teach?
In what format: Was it rather a lecture, an exercise session, personal tutoring, or something else?
(From your description, I presume something lecture-like, and lectures are usually considered more valuable as they require more effort.)
Mention if there any quality evaluation or other quality control (e.g., how well your tutorials were viewed or attended), but do not worry if there are none as this often applies to teaching experience.