En haut, on entend que Dolly jouer.

entendre quelqu'un faire quelque chose (the subject is clear so an infinitive structure is used)
Your version was grammatically right, but this shorter alternative would be preferred

Jeanne : Le dîner estsera/va être prêt endans deux minutes !

Le dîner est prêt en deux minutes = the dinner takes two minutes to be ready
Le dîner sera prêt dans deux minutes = two minutes from now, the dinner will be ready

Jeanne regarde le compte à rebours de la dernière minute du minuteur pour la pizza.

The other way around: she's looking at the last minute "pertaining to" the countdown
A kitchen timer is more accurately called "un minuteur", this word fits better

Ils sont assis tous les trois autours de la table en, partageant la pizza.

autour DE quelque chose
doing something = partageant
"EN partageant" (true gerund) would insist on both actions being simultaneous, but it's not necessary here

Dolly dans son pyjama de Star Wars.

Brands identifying an object generally don't require article, they function as adjectives => un ordinateur Apple, un pyjama Star Wars, un cartable Hello Kitty...

Jeanne feuillette le courrier de manière décontractéenégligemment le courrier.

négligemment = without actually paying attention to it or doing an action with care

Dolly : D'e Holden.

A French-speaking kid would probably say "c'est Holden qui me l'a appris", as the question is "où" and not "de qui", but it's a subtlety

This one would most likely be a "h aspiré" here, so it'd be "de Holden"

Sa sœur l'a enseignée lui a appris.

le = le mot ; lui = to him, to Holden
"enseigner" is a pretty formal word. In everyday conversations, we rather tend to use "apprendre" (which mean both "to teach" and "to learn" but "apprendre quelque chose à quelqu'un" makes it very clear it's "to teach")

Mmm... « Cool ».

Better to directly quote meanings between parentheses for more clarity (that way, a reader immediately understands "cool" is what the word is supposed to mean to her)

Michael : Eh, je suis médiocrnon, moi je suis pas terrible.

Being "médiocre" in French is rather a matter of performances; there are a lot of terms for people who don't look very good: "pas terrible" would fit well

I'd also add "moi" to emphasize the "me vs others" contrast

Dolly : Qui ?

Il lance à Jeanne un regard amusé.

Better not to mix up names: if you want to "francisize" her name as "Jeanne", it's better to stick to it

Dolly : Et moi, est-ce que je suis sexy ?

Same thing here

Jeanne intervient... ¶
¶
¶
Jeanne : Tu le seras un jour, ma puce.

There would be a pronoun here, replacing "jolie"

Mais pas avant un très, très, treès longtemps.

"longtemps" works as an adverb, and therefore doesn't require any article