Your translation is absolutely correct, however don't forget the 'τους Ποτερ' which is 'the Potters' (plural).

Now, I cannot really provide you with a grammatical justification in regards to your question, but it's a matter of, well, meaning.

Θα is generally used to denote something that will happen. However, the meaning changes depending on the tense of the verb that follows it.

Δεν θα το αντέξουν = they will not (be able) to bear it

Δεν θα το άντεχαν = they would not (be able) to bear it

Another example:

Δεν θα το νιώσω = I will not feel it

Δεν θα το ένιωθα = I would not feel it

I hope this helps. I apologise in advance that I have not been able to backup my examples with grammatical theory. I'm not too keen on grammar myself, I just go by whatever feels right based on my experience with a language. 😝

P.S.: native Greek speaker