Some corrections:

• Oh no! Why haven‘t you told/ didn‘t you tell me? -- this is just a simple past expression. No need to use present perfect here.
• And I have never tried/will never try any drugs, I promise. -- both sentences could be correct in different contexts, but the 'I promise' at the end suggests this should be a future tense ("I promise I'll never do that again')
• That will be/is/would be great, Dad! -- but I can't tell you why.

https://learnenglish.britishcouncil.org/engli...

2. We can use the present continuous for plans or arrangements:

I'm playing football tomorrow.

They are coming to see us tomorrow.

We're having a party at Christmas.

Some comments:

• Oh, Mum! I am going to go/will go to a rock concert with some friends. -- better: I am going to a concert (just the present continuous, not "be going to + verb" -- see point 2 above)
• And I am afraid that alcohol or even drugs will/are going to be sold there, Sally. -- I don't know which is better! The sentence is pretty stilted both ways.
• I will/ am going to phone Grandpa and tell him, that we will/are going to come in the afternoon -- I think both are okay
• You know how much Grandpa enjoys seeing/ will enjoy seeing you -- I think both are okay

Further comment:

This dialogue sounds very unnatural and unidiomatic. Not because of the tenses, but because of the absence of contractions. English always tends towards contractions, especially in speech, so this exercise sounds incredibly artificial without them:

• Sally, you know that we're going to visit Grandpa this Friday evening, don‘t you?
• Oh, Mum! I'm going to a rock concert with some friends.
• But I've already phoned Grandpa and told him.
• (...) Mike's already bought the tickets for us.

etc.

Manche Sätze hören sich für mich seh unidiomatisch an:

"If the concert is such an event" event ist hier fehl am Platz. Du meinst wahrscheinlich eher so was wie "is such a big deal".

"I am afraid that alcohol or even drugs will be sold there": Alkohol wird zukindest in den USA praktisch immer bei Konzerten verkauft, allerdings muss man nachweisen, dass man 21 ist. Und ich würde in dem Zusammenhang eher worried denn afraid nehmen.

"I haven't heard a real concert": Ich würde "I haven't been to a real concert" nehmen.