”I don’t think this is joint decision with committee as I receive nothing but praise from the mums on committee.”
The committee have presumably delegated the job of recruitment for this post to her, which is fine. If they are unhappy with the decision and think you should have been temporarily promoted, they would be able to say so.
”I will express my feelings once she tells me officially herself”
Why wait?
”I would of thought they’re would be some kind of expectation for a deputy to step up and I would want her to follow some sort of professional procedure.”
Not at all any kind of “expectation” about deputy stepping up. Sometimes employers want deputies to act up, sometimes they prefer to recruit a direct replacement on a temporary basis. It’s completely up to the employer how they handle it.
What kind of “professional procedure” do you mean? Do you mean it should have been advertised internally so you could apply? That sounds fair, but in circumstances where a decision has clearly been made that you acting up isn’t what they want to do, that would actually be a colossal waste of everyone’s time including your own. The outcome would not have been any different.
The only thing anyone has done wrong here is not communicate well with you.