adults /ˈædʌlts/ or /əˈdʌlts/, plural of adult. The adults arrived first.
The final -s is /s/, not /z/, because it follows /t/. The spelling A-D-U-L-T-S maps straight onto those sounds.
The plural adults is spelled A-D-U-L-T-S and pronounced /ˈædʌlts/ or /əˈdʌlts/. Both forms are standard in UK English, because the noun adult itself carries two accepted stresses: /ˈædʌlt/ with stress on the first syllable, and /əˈdʌlt/ with a reduced first vowel. Keep that pattern and simply add the /s/ ending after the final /t/.
The adults arrived first.
Watch the spelling of the vowel: the u represents the short sound /ʌ/, so avoid the tempting but wrong forms adoults and adaults, which add or shift a vowel that is not there.
UK dictionaries actually allow two stress patterns for the singular: /ˈædʌlt/, with stress on the first syllable, and /əˈdʌlt/, where the first vowel reduces to /ə/ and the stress shifts to the second syllable. Each pattern carries through to the plural, so adults is /ˈædʌlts/ or /əˈdʌlts/. The two forms are alternatives, not different meanings; both are standard in UK English, and either one is acceptable.
Both /ˈædʌlt/ and /əˈdʌlt/ are standard stress patterns for 'adult', so either is acceptable. The plural adds /s/ after the final /t/, giving /ˈædʌlts/ or /əˈdʌlts/.
A spelling trap: the u is /ʌ/, a short vowel, not /uː/ or /ɔː/. Never write adoults or adaults on the model of fault, whose vowel is /ɔː/. Here A-D-U-L-T-S maps directly to /æ-d-ʌ-l-t-s/ or /ə-d-ʌ-l-t-s/.