Mater dicit ventum mox desiturum esse, et nos in horto exspectamus.

It’s an indirect statement (ACI) where:

• ventum = accusative “subject” of the reported clause
• desiturum esse = infinitive part of the reported clause, specifically a future infinitive
  Together: [ventum mox desiturum esse] = that the wind will soon stop.

Because desinere is the present infinitive (“to stop / to be stopping”), not “will stop.”
To express future time in an indirect statement, Latin usually uses:

• future active participle
  • esse
    So desiturum esse conveys future meaning: will stop.

Yes. desiturum is a future active participle agreeing with ventum in:

• case: accusative
• number: singular
• gender: masculine
  So: ventum (m. acc. sg.) + desiturum (m. acc. sg.).

ventum could, in other contexts, be the supine of venīre (ventum = “to come”), but here it’s clearly from ventus (wind) because:

• it’s paired with a participle (desiturum) that agrees with it like a noun it describes
• the meaning “the wind will stop” fits naturally with dēsinere (“cease”)

horto is ablative singular because in with a place meaning where (location) takes the ablative:

• in hortō = in the garden (location)
  If it were motion into the garden, it would be in hortum (accusative).

You typically put nōn inside the indirect statement, before what it negates, e.g.:

• Mater dicit ventum nōn mox desitūrum esse = Mother says the wind will not stop soon.
  Or to negate the whole statement more generally:
• Mater nōn dicit ventum mox desitūrum esse = Mother does not say that the wind will soon stop.

It’s very typical Latin. Common tendencies here are:

• dicit introduces the indirect statement early
• the infinitive chunk often comes toward the end of its clause (… desiturum esse)
• other elements (ventum, mox) are placed where they read smoothly and emphasize the key information before the closing infinitive.